• Energy Research

Recently, the charge carrier transport mechanism of passivating contacts, which feature an ultra-thin oxide layer, has been investigated by studying temperature-dependent current–voltage ( I–V ) characteristics. The measurement revealed that tunneling is the dominant transport path for tunnel oxide passivated contact (TOPCon) with wet chemically grown oxide layer. Furthermore, higher annealing temperatures led to the deterioration of the surface passivation most likely because of excessive pinhole formation. In this contribution, we are going to extend the previous study by analyzing other interfacial oxides as well. We will show that extremely low recombination current densities and low contact resistivity values can be achieved by differently processed TOPCon structures, which are characterized by a predominant tunnel transport as well as one where current flow via pinholes likely predominates. Furthermore, an I–V ( T ) study on solar cells with passivating rear contact reveals that fill factor transitions from a nonlinear to linear behavior when the Si layer turns partially crystalline.

Recombination at the metal-silicon interface is a major barrier to reaching the theoretical power conversion efficiency limits. We present a PL method for evaluating the metal contact recombination current (J0,c) of rear TOPCon metallisation.

Abstract In this work, the efficiency of n -type silicon solar cells with a front side boron-doped emitter and a full-area tunnel oxide passivating electron contact was studied experimentally as a function of wafer thickness W and resistivity ρ b . Conversion efficiencies in the range of 25.0% have been obtained for all variations studied in this work, which cover 150 µm to 400 µm thick wafers and resistivities from 1 Ω cm to 10 Ω cm. We present a detailed cell analysis based on three-dimensional full-area device simulations using the solar cell simulation tool Quokka. We show that the experimental variation of the wafer thickness and resistivity at device level in combination with a detailed simulation study allows the identification of recombination induced loss mechanisms. This is possible because different recombination mechanisms can have a very specific influence on the I-V parameters as a function of W and ρ b . In fact, we identified Shockley-Read-Hall recombination in the c-Si bulk as the source of a significant FF reduction in case of high resistivity Si. This shows that cells made of high resistivity Si are very sensitive to even a weak lifetime limitation in the c-Si bulk. Applying low resistivity 1 Ω cm n -type Si in combination with optimized fabrication processes, we achieved confirmed efficiency values of 25.7%, with a V OC of 725 mV, a FF of 83.3% and a J SC of 42.5 mA/cm 2 . This represents the highest efficiency reported for both-sides contacted c-Si solar cells. Thus, the results presented in this work demonstrate not only the potential of the cell structure, but also that a variation of the wafer thickness and resistivity at device level can provide deep insights into the cell performance.

An increase in market share of very high-efficiency solar cell concepts, such as interdigitated back contact (IBC) solar cells, is typically limited by their high cost due to complex and expensive processing sequences. Implementation of localized laser-doped contacts is a promising route to reduce fabrication costs while maintaining the high-efficiency potential of such solar cell concepts, also offering the potential to use cheaper, upgraded metallurgical silicon which may otherwise be vulnerable to degradation when exposed to traditional high-temperature processing. In this work, we introduce novel test structures and perform numerical simulations to accurately determine the contact properties, namely, the recombination parameter J 0, c down to values of 300 fA cm–2 and contact resistivity ρc down to values of 1 × 10–4 Ω cm2. We analyze the performance of 30 μ m × 30 μ m-sized laser-doped localized contacts prepared using boron and phosphorus spin-on dopants and phosphorus-doped silicon nanoparticle ink. The laser-doped contacts characterized in this work allow for 23.7% efficient IBC solar cells as we demonstrate by numerical.

Perimeter recombination causes significant efficiency loss in solar cells. This paper presents a method to quantify perimeter recombination via luminescence imaging for silicon solar cells embedded within the wafer. The validity of the method is discussed and verified via 2-D semiconductor simulation. We demonstrate the method to be sufficiently sensitive in that it can quantify perimeter recombination even in a solar cell where no obvious deviation from ideality is observed in the current–voltage (J–V) curve.

Laser doping of silicon is a complex process involving thermal effects and interactions between different materials far from equilibrium and over a short period. In this paper, diffused samples capped with different dielectric films (including bare surfaces) are processed using laser pulses of 20-400 ns duration and characterized by photoluminescence (PL) imaging to study the degradation of the electronic properties of the processed regions. This way, without the interference of a dopant precursor, the thermal and dielectric effects are separately investigated. It is found that the thermal effects (melting and recrystallization of the silicon) do not lead to significant damage and additional recombination, provided no severe silicon evaporation occurs. However, when a dielectric film is present, a considerable increase in recombination is observed, irrespective of laser parameters, indicating the formation of additional defects. The magnitude of the increase in recombination varies substantially, depending on the dielectric used. Repeated pulses appear to repair silicon damage introduced by the first pulse or pulses for long pulse durations but result in a slight degradation for short pulse durations. Combining the PL results and four-point probe measurement of laser-doped samples, it is demonstrated that both high dopant incorporation (sufficient silicon melting) and low recombination can, in principle, be achieved, particularly when samples are processed using long pulse durations and small pulse distances.

AbstractHigh efficiency solar cell concepts typically depend upon highly localized processing technologies, such as partial rear contacts. In practice, characterizing these techniques is challenging and it has become increasingly popular to employ analytical expressions that fit the local surface recombination velocity (SRV) to area-averaged measurements of regularly arrayed test structures. However, this approach has numerous limitations imposed by the assumptions of the model and as a consequence significant inaccuracies can result. We present an alternative approach with improved accuracy and generality, which makes use of the recently developed fast 2D/3D device simulator Quokka. Quokka has been enhanced to predict luminescence as well as photoconductance (PC). Thus the recombination activity of the locally processed feature can be iteratively found to fit the measurement result of a test sample. The power of this technique is demonstrated by the derivation of the recombination current density as a function of the excess carrier density directly at the local feature from area-averaged photoluminescence (PL) signals. We apply the method to different laser-doped samples, which reveal accurate and consistent trends for a range of pitches and injection levels as well as a low sensitivity to the uncertainty of input parameters.