• Energy Research

AbstractTime‐resolved photoluminescence (TRPL) measurements are one of the key metrics available to determine the minority‐carrier lifetime in the absorber layer of direct band gap photovoltaic devices. Direct measurement of the minority‐carrier lifetime is essential to understanding the impact of changes in deposition and processing on material quality. Unfortunately, the TRPL signal is determined by a complex convolution of multiple physical factors including bulk carrier lifetime, interface recombination velocity, electric field, doping density, photo‐excited carrier density, and carrier mobility. To gain clarity, we have used numerical simulations to analyze the carrier dynamics after a CdTe device is illuminated with a short light pulse. After the light pulse, the photo‐generated carriers undergo complex dynamics including drift, diffusion, interface, and bulk recombination. In this work, we develop a new formalism that enables much greater insight into which factors dominate the TRPL decay dynamics. By breaking down the carrier dynamics into drift, diffusion, and recombination terms, we have developed six‐factor, four‐factor, and two‐factor analyses that provide clear understanding of which physical factors dominate the decay dynamics under various conditions and at different times during the decay. We show that in a typical CdTe device under the typical experimental conditions used in our laboratories, the faster part of the decay is dominated by charge separation, whereas the slower part is dominated by carrier recombination. Therefore, under the conditions investigated in this study, the slower part of the decay is a better parameter to explain the defect density in the CdTe layer. Copyright © 2013 John Wiley & Sons, Ltd.

Advancing CdTe solar cell efficiency requires improving the open-circuit voltage $\rm{(V_{{\rm{OC}}})}$ above 900 mV. This requires long carrier lifetime, high hole density, and high-quality interfaces, where the interface recombination velocity is less than about 104 cm/s. Using CdTe single crystals as a model system, we report on CdTe/CdS electrical and structural interface properties in devices that produce open-circuit voltage exceeding 950 mV.

As the development of kesterite solar cells accelerates, the bottlenecks in device performance need to be identified and ways for their circumvention defined and developed. In this work, we use 2-dimensional (2D) numerical simulations to explore possible reasons for low open-circuit voltage (Voc) in Cu2(Zn,Sn)Se4 (CZTSe) solar cells. High defect density in the CZTSe absorber and at the CZTSe/CdS interface can be significant reasons for Voc deficit, but they do not explain all of the losses observed experimentally. Local deviation from stoichiometry could create secondary phases with a lower band gap compared to the absorber. These secondary phases can be severely harmful to Voc if located in the vicinity of the heterointerface and along the grain boundaries.

Achieving higher photovoltaic efficiency in single-junction devices is becoming increasingly difficult, but tandem modules offer the possibility of significant efficiency improvements. Device modeling shows that four-terminal CdTe/Si tandem solar modules offer the prospect of 25%–30% module efficiency, and technoeconomic analysis predicts that these efficiency gains can be realized at costs per Watt that are competitive with CdTe and Si single junction alternatives. The cost per Watt of the modeled tandems is lower than crystalline silicon, but slightly higher than CdTe alone. However, these higher power modules reduce area-related balance of system costs, providing increased value especially in area-constrained applications. This avenue for high-efficiency photovoltaics enables improved performance on a near-term timeframe, as well as a path to further reduced levelized cost of electricity as module and cell processes continue to advance.

Low open-circuit voltages (850-870 mV), due to excessive bulk and surface recombination, currently limit CdTe photovoltaic efficiencies. Here, we study surface recombination in single crystals with single-photon excitation time-resolved photoluminescence (1PE-TRPL) to measure minority carrier lifetimes. Typically, minority carrier lifetimes of untreated undoped CdTe material as measured by 1PE-TRPL are ~100 ps or less, even though their bulk lifetimes as measured by two-photon excitation TRPL can reach 100 ns. Such short 1PE-TRPL lifetimes indicate very high surface recombination velocities exceeding 100 000 cm/s. Here, we examine treatments that can reduce surface recombination and discuss different ways of evaluating their efficacy.

In this study, Deep Level Transient Spectroscopy (DLTS) measurements have been performed on Cu(In,Ga)Se2 (CIGS) solar cells from an inline co-evaporation system. The focus of this investigation is directed on the effect of rubidium-fluoride (RbF)-post-deposition treatment (PDT) on the defects in the CIGS absorber layer. Different traps can be identified and their properties are calculated. Herein, different methods of evaluations have been used to verify the results. Specifically, one minority trap around 400 meV was found to show a significant reduction of the trap density due to the alkali treatment. In contrast, a majority trap at approximately 600 meV is unaffected.

We study the effects of Cu composition on the CdTe/ZnTe:Cu back contact and the bulk CdTe. For the back contact, its potential barrier decreases with Cu concentration while its saturation current density increases. For the bulk CdTe, the hole density increases with Cu concentration. We identify a Cu-related deep level at ∼0.55 eV whose concentration is significant when the Cu concentration is high. The device performance, which initially improves with Cu concentration then decreases, reflects the interplay between the positive influences (reducing the back contact potential barrier while increasing the saturation current density of the back contact and hole density in CdTe bulk) and negative influences (increasing deep levels in CdTe) of Cu.

AbstractWe improved the efficiency of ultra‐thin (0.49‐μm‐thick) Cu(In,Ga)Se2 solar cells to 15.2% (officially measured). To achieve these results, we modified growth conditions from the 3‐stage process but did not add post‐deposition treatments or additional material layers. The increase in device efficiency is attributed to a steeper Ga gradient in the CIGS with higher Ga content near the Mo back contact, which can hinder electron‐hole recombination at the interface. We discuss device measurements and film characterization for ultra‐thin CIGS. Modeling is presented that shows the route to even higher efficiencies for devices with CIGS thicknesses of 0.5 μm.