Relationships between diffusion parameters and phosphorus precipitation during the POCl3 diffusion process

The POCl3 diffusion process is still a common way to create the pn-junction of Si solar cells. Concerning the screen-printing process, it is necessary to find a compromise between low emitter recombination, low contact resistance and high lateral conductivity. The formation of a homogeneous emitter during the POCl3 diffusion process depends on several diffusion parameters, including duration, temperature and gas flow. This primarily controls the growth of the highly doped phosphosilicate glass (PSG) layer, which acts as a dopant source during the diffusion process. Detailed investigations of the PSG layer have shown a distinct correlation between the process gas flows and the composition of the PSG layer. Specifically, in this research we examine the influence of phosphorus precipitation at the PSG/Si interface. Furthermore, we show the influence of phosphorus precipitation during the pre-deposition phase on the passivation quality of the corresponding emitter. In a second step, we use the results to create emitters with a reduced density of phosphorus precipitates. In a last step, the optimized emitter structure was transferred to screen-printed solar cell processes, whereby efficiencies up to 19.4% abs. were achieved on monocrystalline p-type Cz material with full area Al-BSF rear side

Free energy loss analysis decomposition of the power voltage characteristic of an interdigitated back contact solar cell

The current voltage (JV) curve decomposition is a very powerful tool for interpreting simulation results but also measurements. However, drawing conclusions from it about power losses by simply multiplying by the solar cell voltage V is firstly incomplete because it overlooks power losses during transport and secondly incorrect because all free carriers in the solar cell do not carry an energy equal to q·V. Transport losses are therefore classically introduced separately through the series resistance that incorrectly assumes a monopolar transport in the solar cell. The two issues are addressed in the recently developed Free Energy Loss Analysis (FELA) leading to the rigorous power voltage (PV) curve decomposition. However, interpretation rules as well as links to the classical approach are still missing to make FELA a useful tool for solar cell development. It is the goal of this paper to make the first steps in this direction by attempting to explain the FELA losses dependence to voltage and comparing them to classically described power losses. Good correlation is shown between recombination current and power loss except at high voltage, and a link is established between FELA majority carrier Joule losses and series resistance losses. The FELA minority carrier Joule losses that are a consequence of collection but also of recombination remain undescribed classically and deserve further investigation

Discussion and simulation about the evaluation of the emitter series resistance

The emitter series resistances (Rs,emi) can be extracted from experimentally measured JV curves using the two-light intensity method (TLIM) but it can also be calculated from the emitter geometry using analytical formulas and finally also computed with arbitrary precision using finite element simulation (FES). On the one hand formulas and FES consider Rs,emi as distributed, on the other hand the TLIM assumes Rs,emi not to be distributed. The derivation of formulas for a lumped Rs,emi assumes a spatially uniform current density source, which is the case in short circuit condition (Jsc), less at maximum power point (mpp) and is wrong at open circuit. We compare at mpp the results of TLIM, analytical formulas and FES for which the current density source is a 1D simulated JV curve. In the case of a low/high sheet resistance homogeneous emitter, but also for a selective emitter, these methods agree well and the impact on cell efficiency is particularly small. This is partially explained by the fact that the voltage drop and so the spatial distribution of the current density source over the emitter is small at mpp. We also clarify many issues about the various methods used and discuss the limitation of not taking into account Joule losses induced by diffusion

On the characteristics of the doping profile under local metal contacts

In many solar cell concepts, the recombination at local contacts is a bottleneck for the efficiency. Therefore, an optimized doping profile underneath the metal contact would improve the cell performance. We investigate the saturation current density (J0e,met) value of various doping profiles by TCAD simulation and showed that lowest J0e,met values are obtained for profiles with a surface concentration Ns > 5·1020 cm−3 as a consequence of the Pauli blocking and almost independently of the junction depth xj. For profiles with lower Ns we could show an approximate proportionality between J0e,met and the sheet resistance (Rsheet) making the recombination performance of these profiles quasi-independent of the profile shape. Therefore, profiles with even lower value of Rsheet as presently used, typically s > 1020 cm−3 could allow to reach even lower J0e,met, typically 2. In general Auger recombination is very low (2 for Rsheet > 5 Ω/sqr) and does not play a role in the optimal profile shape of the emitter.publishe

Quantitative interpretation of light beam induced current contrast profiles : back side surface influence

The quantitative interpretation of a Light Beam Induced Current (LBIC) contrast profile of a grain boundary (GB) allows the extraction of the ‘recombination strength’ of the GB, characterized by its effective surface recombination velocity (Seff), and of the diffusion length (Ldiff) of the neighboring grains. The previous fitting model developed by Donolato [1] assumed an infinite wafer thickness (h) that restricts its validity to cases where Ldiff<<500 μm) the present extension can provide a local estimation of Sb that reflects the back side electrical quality. The quantitative evaluation of Ldiff and Seff is very useful for e.g. the evaluation of the effectiveness of a hydrogenation or gettering step in a solar cell process

Solar cell improvement by using a multi busbar design as front electrode

The demand for low-priced solar cells with higher efficiencies becomes more necessary to reach grid parity. An optimized solar cell design which uses the same equipment as state of the art solar cells could be easily implemented into solar industry. In this paper an approach for a front side design is discussed, using more busbars than the widely used three busbar design for the solar cell front electrode. A simulation program based on the two-diode model was used summing up the series resistances of each contributor in a module optimizing the number and geometry of wires needed. The simulations reveal that the advantages of the multi busbar design originate from a reduction of effective finger length, opening up new possibilities for the cell design. It is demonstrated that the multi busbar solar cell design can increase the module efficiency by 0.5%abs and a reduction in the consumption of silver of over 89% can be achieved by using seed and plate techniques. A significant surplus in short circuit current can be realized by using round wires

Quantitative interpretation of light beam induced current contrast profiles for differing diffusion lengths on either side of a grain boundary

The quantitative interpretation of a Light Beam Induced Current (LBIC) contrast profile (LBIC signal normalized to the signal infinitely far from the grain boundary) of a Grain Boundary (GB) allows the estimation of the diffusion length in the neighboring grains (left grain L1, right grain L2,) as well as the recombination strength of the GB characterized by its equivalent Surface Recombination Velocity (SRV) vs. The quantitative evaluation of L1, L2 and vs is very useful regarding e.g. the evaluation of the effectiveness of a hydrogenation step in a solar cell process. For this purpose, we developed a direct fitting procedure based on a particular solution of the minority carrier diffusion equation with suitable boundary conditions. This theory, initially developed by C. Donolato for analyzing EBIC and/or LBIC contrast profiles, had the limitation to assume the same diffusion length (Ldiff) for both neighboring grains which cannot account for non symmetrical contrast profile due to differing Ldiff and can thus lead to erroneous evaluation of Ldiff , in particular when the SRV of the GB is low. The present contribution fixes this problem by generalizing Donolato’s theory for differing diffusion lengths on either side of the GB, and so allows non symmetrical profiles to be investigated

Quantitative interpretation of light beam induced current contrast profiles : evaluating the influence of a nearby grain boundary

The quantitative interpretation of a Light Beam Induced Current (LBIC) contrast profile (LBIC signal normalized to the signal infinitely far from the grain boundary) of a Grain Boundary (GB) allows the estimation of the diffusion length in the neighboring grains (left grain L1, right grain L2,) as well as the recombination strength of the GB characterized by its equivalent Surface Recombination Velocity (SRV) vs. The quantitative evaluation of L1, L2 and vs is very useful regarding e.g. the evaluation of the effectiveness of a hydrogenation step in a solar cell process. For this purpose, we developed a direct fitting procedure based on a particular solution of the minority carrier diffusion equation with suitable boundary conditions. This theory, initially developed by C. Donolato for analyzing EBIC and/or LBIC contrast profiles, had the limitation to assume the same diffusion length (Ldiff) for both neighboring grains which cannot account for non symmetrical contrast profile due to differing Ldiff and can thus lead to erroneous evaluation of Ldiff , in particular when the SRV of the GB is low. The present contribution fixes this problem by generalizing Donolato s theory for differing diffusion lengths on either side of the GB, and so allows non symmetrical profiles to be investigated

Injection in light beam induced current systems : An analytical model

In LBIC systems, the evaluation of the injection level is necessary when operating in low injection for defect recombination studies or in defined standard illumination conditions (one or several suns for concentrator applications) for quantum efficiency evaluation. We demonstrate in this contribution that evaluating the laser beam induced injection based on uniform illumination condition can lead to several decades of error because of the lateral carrier diffusion. Based on a parallel beam approximation, we propose here an analytical model to evaluate the maximum of injection of a laser with its parametrization valid for most LBIC system settings and material quality. State of the art high resolution LBIC (HR-LBIC) systems have so sharply focused laser beams that the beam divergence cannot be neglected anymore in the injection calculation. Although providing a quantitative criterion to determine whether the beam divergence can be neglected, we provide a more advanced model for describing the injection of the laser that includes beam divergence.publishe