Understanding the Solar Cell Contacts With Atmospheric Screen-printed Copper

Front-contact passivation through 2D/3D perovskite heterojunctions enables efficient bifacial perovskite/silicon tandem solar cells

Recently, high efficiency n-type crystalline Si cells made with the screen printed Ag/Al metallization have received considerable attention. We report here our microstructural investigations of the critical interfacial region between the front-side contact and the Si wafer of n-type cells fired under progressively higher temperatures. Our study revealed that the key characteristic microstructures of the interfacial region changed from one with a large fraction of residual SiNx, to one consisting of a thin glass layer with nano-Ag colloids, and finally to one decorated with Ag and Ag/Al crystallites attached to the emitter surface for cells with under-, optimally-, and over-fired conditions, respectively. We did not find any Al-Si eutectic layer on the emitter surface that would support a silicon dissolution and re-growth mechanism, which is operative in the back surface field formation process for the Al back contact of p-type industrial solar cells. The presence of the SiNx antireflection coating has likely altered the chemistry between Si and Al significantly. The observed microstructures lead us to conclude that the main current conduction mechanism in optimally-fired n-type cells is tunneling through those areas of thin interfacial glass containing nano-Ag colloids. This mechanism is similar to the current conduction model we have proposed previously for optimally-fired p-type crystalline Si solar cells. We believe that the intrusion of Ag/Al (and/or Ag) crystallites into the p+-Si emitter in over-fired cells is one of the major sources of metallization-induced recombination losses, which degrades cell performance.

Investigation of Laser-fired Rear-side Point Contacts of Laser-crystallized Silicon Thin-film Solar Cells by Conductive Probe Atomic Force Microscopy

The deposition of the chalcopyrite light absorbers onto the back contact of thin films solar cells involves the adsorption of Se on Mo surfaces, to form an intermediate MoSe2 layer. Together with Se and the other elements composing the absorber, also impurities like oxygen and sodium are present in common processing technologies. In order to promote the understanding of atomic mechanisms and basic thermodynamic parameters related to deposition, we predict the adsorption energies and most stable adsorption sites of Se, O and Na on the Mo(110) surface using first principles calculations, based on density functional theory. We discuss implications for kinetics on the surface.

Organohalide perovskites have emerged as a class of materials with a unique combination of optoelectronic properties, suitable for a plethora of applications ranging from solar cells to photoelectrochemical tandem cells, to lasing and lighting. Theoretical and computational modeling can deliver an hitherto inaccessible atomistic view of the crucial material properties and heterointerfaces ruling the operational mechanisms in all these devices. Here we present a unified view of recent activity in the computational modeling of interfaces relevant to perovskites solar cells. The performance of the proposed simulation toolbox along with the fundamental modeling strategies are illustrated using selected examples of relevant materials and representative interfaces. In particular, we discuss interfaces between the prototype methylammonium lead iodide perovskite with TiO2 and ZnO semiconductors (acting as electron selective contacts in solar cells), exploring different surface terminations and doping by chloride ions. Also the effect of defects at the interface with TiO2 is analyzed and their impact on solar cell performance is discussed. Finally, the heterogeneous interface between methylammonium lead iodide and water is analyzed, revealing dynamical hints on the perovskite degradation by water.

In this paper we will study the formation and characteristics of nickel silicide compounds and the subsequent specific contact resistivity (Ω.cm2) during the formation of an ohmic contact in solar cells. Using Transmission Line Method (TLM) we will analyse the electrical characteristics of the formed nickel silicides at different sintering temperatures and durations. In each sintering condition tested, the relationship between the total resistance and the distance between the contact pads on the fabricated test structures is measured and graphed. From these graphs the contact resistance (Ω) and specific contact resistivity (Ω.cm2) of the metallisation stack is calculated and tabulated. We conclude the paper by showing the optimum sintering condition of 350°C for three minutes for laser doped solar cells which yields negligible contact resistance.

Chapter 8 - Cu(In 1− xGa x)Se 2 and CuIn(Se 1− xS x) 2 Thin Film Solar Cells

Novel hole selective CrOx contact for dopant-free back contact silicon solar cells

This study addresses the design of radio frequency (rf) magnetron sputtered aluminum doped zinc oxide (ZnO:Al) front contacts for silicon thin-film solar cells. Optimized films comprise high conductivity and transparency, as well as a surface topography trapping the light within the photovoltaically active layers. We have investigated the influence of the doping level of the target as well as the substrate temperature during sputter deposition on the ZnO:Al properties. The aluminum content in the target influences the transmission in the near infrared (NIR), the conductivity as well as the film growth of the ZnO:Al layer. The latter affects the surface topography which develops during wet-chemical etching in diluted hydrochloric acid. Depending on aluminum content in the target and heater temperature three different regimes of etching behavior have been identified. We have applied the ZnO:Al films as front contacts in thin-film silicon solar cells to study their light trapping ability. While high transparency is a prerequisite, the light trapping has been improved using front contacts with a surface topography consisting of relatively uniformly dispersed craters. We have identified low amount of target doping and high substrate temperatures as sputter parameters enabling high cell currents. Short-circuit current densities of up to 26.8 mA/cm² have been realized in μc-Si:H single junction cell with absorber layer thickness of 1.9 μm.

Indium Tin Oxide (ITO)-free carrier-selective contact for crystalline silicon solar cells is developed by the integration of a micro-grid electrode.

[This corrects the article DOI: 10.1039/D3NA00811H.].

Atomic layer deposition (ALD) of Aluminum oxide (Al2O3) is employed to optimize the back contact of thin film CdTe solar cells. Al2O3 layers with a thickness of 0.5 nm to 5 nm are tested, and an improved efficiency, up to 12.1%, is found with the 1 nm Al2O3 deposition, compared with the efficiency of 10.7% without Al2O3 modification. The performance improvement stems from the surface modification that optimizes the rectification and tunneling of back contact. The current-voltage analysis indicates that the back contact with 1 nm Al2O3 maintains large tunneling leakage current and improves the filled factor of CdTe cells through the rectification effect. XPS and capacitance-voltage electrical measurement analysis show that the ALD-Al2O3 modification layer features a desired low-density of interface state of 8 × 1010 cm−2 by estimation.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was studied as the back contact of Cadmium telluride (CdTe) solar cells and was compared with conventional Cu-based back contact. A series of PEDOT:PSS aqueous solutions with different conductivities were spin coated onto the glass/SnO2:F/SnO2/CdS/CdTe structures as back contact, and the PEDOT:PSS conductivity dependence of device performance was studied. It was found that PEDOT:PSS back contact with higher conductivity produces devices with lower series resistance and higher shunt resistance, leading to higher fill factor and higher device efficiencies. As the conductivity of PEDOT:PSS increased from 0.03 to 0.24 S/cm, the efficiency of the solar cell increased from 2.7 to 5.1 %. Methanol cleaning also played an important role in increasing the device performance. The efficiency of our best device with PEDOT:PSS back contact has reached 9.1 %, approaching those with conventional Cu/Au back contact (12.5 %).

Transition metal oxides with a high work function can be used as hole-selective materials for c-Si solar cells. WOx and MoOx can have work function as high as >6 eV which has attracted increasing interests in the c-Si PV community. Apart from the optical and electrical properties of such materials, their thermal stability is of crucial importance for the application of these contacts in high-efficiency silicon solar cells. This work investigates the thermal stability of WOx and MoOx films with and without a-Si:H interface passivation layer using in situ X-ray photoelectron spectroscopy (XPS). The samples were annealed and measured inside the XPS chamber under high vacuum in the 300 K to ≥ 900 K temperature range. This semi-quantitative XPS analysis shows that the oxidisation state of tungsten reduces from +6 to +5 for annealing temperatures > 600 K, while molybdenum is significantly less stable and its oxidation state reduces for all investigated annealing temperatures. When using a-Si:H interface passivation layer it is found that hydrogen diffusing from the a-Si:H enhances the degradation of the metal’s oxidation state at high temperature. Besides, the passivation level of the film stack a-Si:H/WOx showed a maximum lifetime of 3.5 ms (at 1×1015 cm-3) after annealing at 500 K, and for the case of a-Si:H/MoOx annealing resulted in a decrease of the minority carrier lifetime.

Magnetic field measurements and numerical simulation of the current distribution in the emitter region of solar cells