We present an approach for the determination of the locally averaged recombination parameter j(0,met) assigned to the metal contacts of a solar cell. A set of samples with test fields combined with an interpolation scheme allows the precise prediction of expected local photoluminescence imaging (PLi) intensities for virtually non-metallized sample areas with a standard deviation of 0.7%. The ratio of the actually measured PLi signal in a metallized test field with respect to the predicted signal of the virtually non-metallized test field then serves as an input for numerical simulations for the extraction of j(0,met). We find a clear correlation of the locally determined j(0,met) with both, the local peak firing temperature measured in-situ by a thermal imaging system installed in the firing furnace and the local sheet resistance of the diffused n(+)-region. These results demonstrate the high capability and accuracy of our approach for the uniformity characterization of metallization-induced recombination losses

Low‐cost and high‐efficiency tandem solar cells are promising candidates for a future industrial mass production. Nowadays, the passivated emitter and rear cell (PERC) technology makes up the major market share; therefore, it is an attractive option to use the PERC technology as bottom cell concept for a perovskite–silicon tandem device. Long‐term optimization of the PERC technology led to highly efficient, low‐cost, and mature devices. For PERC‐like bottom cells, mainly an adapted front‐side design is needed: Design constrains of a PERC single junction are relaxed to some extent, because such an updated PERC bottom cell only needs to absorb long wavelength photons, transports about half the current and, in a monolithic tandem, lateral transport, and the use of firing‐through local silver contacts is not a mandatory prerequisite. Consequently, to make PERC suitable for tandem application, a systematic reevaluation of the current PERC technology is performed here considering five different front‐side concepts. We investigate locally contacted and full‐area transparent conducting oxide (TCO)‐based interconnection concepts for PERC, as well as the full‐area tunnel oxide passivating contact (TOPCon). Our simulation work elaborates on the advantages and physical constrains of each concept and gives guidelines for the optimization of the phosphorus diffused emitter and front interconnection layers. We conclude that both full‐area and locally contacted front‐side concepts are promising candidates for achieving tandem cell efficiencies of about 30%.

We have developed a fast firing oven (FFO) firing profile that mitigates light and elevated temperature induced degradation (LeTID) in boron doped passivated emitter and rear cells (PERC), made from high-performance multicrystalline silicon (lip mc) and mono-cast material. During LeTID testing, the highest degradation in relative efficiency in the samples fired with this profile is only -2%(rel) < Delta eta < -1%(rel) depending on the cell, compared to -5%(rel) < Delta(eta) < 6%(rel) in samples fired with the standard profile. To show that the benefit of this technique can be transferred to modules, the influence of the temperature profile during lamination was investigated. The effect of LeTID mitigation persisted after the simulated lamination process. This new firing profile did not significantly influence the initial efficiencies of the cells that have been fired with it compared to samples fired with a standard firing profile (-0.2%(abs) <Delta eta<+0.1%(abs)). Since the new FFO profile is only a slight alteration to a standard profile, it can be easily integrated into existing production lines. The altered firing profile therefore seems to be a promising way to handle the LeTID challenge and ensure highest cell efficiencies throughout the whole lifecycle of modules made from mono-cast, hp mc, and probably other types of silicon solar cells in the field. The mono-cast wafers used in this work were grown with the seed manipulation for artificially controlled defects technique (SMART). It solves the challenge of grain boundaries growing inward from the crucible wall by introducing dislocation clusters near the wall that prevent this growth. We show that the efficiency of cells that were made with this technique (21.4%) is similar to that of magnetically grown Czochralski (mCz) cells (21.6%) which were otherwise produced identically. A third group of hp mc cells which have also been identically produced except for an acidic texture instead of random pyramids, however only achieved 19.5% efficiency. Therefore, using a standard PERC process, SMART mono material is clearly superior to hp mc material and is similar to mCz silicon. We also show that mCz cells do not show LeTID degradation.

We apply phosphorus-doped silicon nitride (SiN X :P) layers on the front surface of p -type Czochralski-grown silicon (Cz-Si) passivated emitter and rear cells (PERC). The layers are formed using industrial-type plasma-enhanced chemical vapor deposition. They provide excellent surface passivation with implied open-circuit voltages $\text{i}V_{\text{OC}}$ up to 695 mV and similar optical properties as conventional undoped SiN X anti-reflection layers with $\text{i}V_{\text{OC}}$ peaking at 685 mV. The emitter dark saturation current density at the passivated textured surface is $j_{\rm 0e}$ ≈ 40 fA/cm² for a lowly-doped emitter with sheet resistance $R_{\text{sh}}$ ≈ 160 Ω/sq. For realizing laser-doped selective emitters (LDSE), local laser processing is applied to introduce additional dopants from the SiN X :P layer into the silicon. Thereby, highly doped areas are formed in which $R_{\text{sh}}$ is decreased down to 55 Ω/sq. We refer to this as the nPassDop approach, which provides both local high doping and local structuring of the anti-reflection coating in a single process step. Low contact resistivity of around 1.5 mΩcm 2 is measured for screen-printed and fired silver contacts on the locally laser-doped areas. A first proof of concept of large-area p -type Cz-Si PERC solar cells with front side $\text{SiN}_{\text{X}}\text{:P}$ passivation and LDSE structure yield energy conversion efficiencies up to 20.9%.

In this work, we present an approach to model metallization-induced recombination losses j0,met of screenprinted fire-through metallization pastes, which are used e.g. for the contacts on the front side of passivated emitter and rear cells (PERC). Modelling is based on the local phosphorus emitter doping profile and the local microstructure of the interface between metal contact and silicon surface, which is characterized during a quantitative microstructural analysis using the scanning electron microscope (SEM). By comparing the modelled j0,met to measured j0,met results determined at the same position of both, the microstructural analysis and emitter doping profile, we find that in most cases recombination is dominated by the etch back of the emitter by the glass frit rather than by penetration of crystallites into the doped region. However, for crystallites protruding deep into the emitter or rather up to the junction, recombination at the crystallites becomes significant as well. Our modelled results yield indications that the etch depth into the emitter doping profile by the glass frit contained in the metallization paste, is sensitive to the emitter doping profile itself.

Herein, the high temperature stack oxidation (HiTSOx) approach for the fabrication of passivated emitter and rear cells (PERCs) is investigated. This approach features a combination of phosphorus oxychloride (POCl3) diffusion shortened to the phosphosilicate glass (PSG) deposition phase as well as high temperature thermal oxidation using stacked wafers. During the latter thermal oxidation, the incorporated phosphorus is redistributed and diffuses deeper into the silicon wafer. The simultaneously growing thermal dioxide serves as a passivation layer. Due to the use of stacked wafers, the throughput of the HiTSOx approach is three times higher in comparison to state‐of‐the‐art oxidation at moderate temperature. Applying a busbarless metallization layout, a median energy conversion efficiency of η = 22.2% is achieved for the HiTSOx approach, being similar to the performance of the reference group with state‐of‐the‐art PERC processing also with η = 22.2%. Despite stacking of the wafers during the thermal oxidation, an excellent homogeneity of the oxide layer thickness of ±1 nm over the wafer surface is found, whereas the passivation quality features dark saturation current densities j0e as low as j0e = (30 ± 3) fA cm−2 at emitter sheet resistances Rsh = (199 ± 6) Ω sq−1.

Metallization induced recombination losses are one dominant loss mechanism for current industrial solar cells. A precise determination of these losses is important for contacting technology optimization, as well as precise solar cell modeling. Usually, for state-of-the-art approaches to determine j 0,met , it is assumed that the samples itself exhibit spatially uniform properties (e.g., carrier lifetime or sheet resistance) or that the used reference samples are identical to the metallized samples. Finally, in most cases, only one global j 0,met -value for the entire sample is given, neglecting possible spatial inhomogeneities. In this article, we mostly eliminate the necessity for the assumptions of perfect sample homogeneity by means of an interpolation scheme of the photoluminescence (PL) signal. Thereby, we can predict the PL signal of a virtually nonmetallized test field with a relative standard deviation of about σ ≈ 0.7%. Additionally, we determine j 0,met for specific test fields at different positions on the sample and correlate the results to the local emitter sheet resistance R sh , the local peak firing temperature of the sample during the fast firing process T peak , and the test field finger spacing d. For our samples, a reduction of d from d = 1000 μm to d = 200 μm leads to a reduction of j 0,met by up to 18%. This strong effect is physically unexpected and so far not considered by the state-of-the-art approach, frequently performed in the photovoltaic community. Further, we perform a sensitivity and error analysis which reveals that we are able to determine j 0,met within an estimated accuracy between 15% and 18%.

Herein, boron‐doped cast‐monocrystalline silicon wafers that have been fabricated using the Seed Manipulation for ARtificially controlled defect Technique (SMART mono‐Si) are examined. Their suitability for passivated emitter and rear cell (PERC) fabrication is investigated. Applying a zero busbar layout energy conversion efficiencies of η = 21.9% for SMART mono‐Si, η = 22.2% for gallium‐doped Cz‐Si (Cz‐Si:Ga), and η = 22.3% for boron‐doped Cz‐Si (Cz‐Si:B) are achieved at similar doping levels between 0.7 Ω cm ≤ ρB ≤ 1.0 Ω cm. Therefore, SMART mono‐Si PERCs show almost the same performance as Cz‐Si PERCs. Apart from the performance of SMART mono‐Si PERCs, the minority charge carrier bulk lifetime τB of the SMART mono‐Si wafers after different high‐temperature process steps in the PERC process flow is investigated. After emitter formation, this analysis confirms the high material quality of SMART mono‐Si yielding τB ≈ 1.3 ms at an injection level of Δn = 1015 cm−3. The bulk lifetime after firing is similar to the level determined for mCz‐Si:B and Cz‐Si:Ga reference wafers of similar doping level.

We investigate the effective surface recombination velocity S-eff of alkaline textured, Phosphorus-diffused and thermal SiO2/SiNx passivated surfaces with an emphasis on the impact of the thermal oxidation temperature. The application of a recent calibration procedure for the carrier lifetime measurements enables a precise determination of the dark saturation current density. The experimental results include 25 diffusion/oxidation process combinations that cover a wide range of final surface concentration levels N-s between 3.10(20) cm(-3) and 1.10(19) cm(-3), using oxidation temperatures T-ox from 650 degrees C to 900 degrees C. This yields a data set that enables a revision of the commonly applied parameterization of the effective surface recombination velocity of this passivation scheme using numerical simulation. In addition, the impact of fixed surface charges is modeled for separating field effect and chemical passivation properties. Also, the role of the oxidation temperature on the passivation quality is investigated.

Current studies reveal the expectation that photovoltaic (PV) energy conversion will become the front-runner technology to stem against the extent of global warming by the middle of this century. In 2019, the passivated emitter and rear cell (PERC) design has taken over the majority of global photovoltaic solar cell production. The objective of this paper is to review the fundamental physics of the underlying cell architecture, its development over the past few decades to an industry main stream product, as well as an in-depth characterization of current cells and the future potential of the device structure. The early development of PERCs was set by an intriguing series of improvements starting in 1989 and resulting in a long-standing energy conversion efficiency record of 25.0% set up in 1999. It took a decade of intense technological development to implement this structure as an upgrade to existing production lines and another decade to increase the efficiency of industrially manufactured cells to over 22%. Our analysis of state-of-the-art large-area screen-printed PERCs is based on the pilot-line technology in the Photovoltaic Technology Evaluation Center at the Fraunhofer ISE, which is assumed to be representative of current state-of-the art cell processing. The main recent cell efficiency improvements have been achieved thanks to fine line metallization taking advantage of the high quality emitter formation and passivation and to improvements in material quality. In order to enhance the energy yield of the PV modules, innovations in interconnection technology like multibusbar and shingling technology as well as bifaciality are supported by PERC developments. Over the years, ongoing improvements have been made in the understanding of PERCs by analytical and numerical modeling of these devices. We show a study based on 3D numerical modeling and an extrapolation of the PERC device structure and technology to achieve an efficiency of 26%. This result surpasses earlier investigations due to the combination of technology components, as further improved front contact and emitter design as well as rear passivation and mirrors. We expect that PERCs can also play a strong role at the bottom of multijunction solar cells and will defend a strong position in global PV production beyond the end of the now starting decade.

We present two approaches for high-accuracy aligning of patterning processes with each other when fabricating solar cells. We introduce the approaches on the example of two different patterning processes of which one is adjustable (laser process) and one is not adjustable (screen-printing process). The basic idea is to measure the coordinates of the applied structures of each involved patterning process at discrete grid points with respect to a reference coordinate system. We chose the grid points such that they completely define the final cell pattern. Then, we adjust the grid point coordinates of one of the patterning processes (the laser process) according to the pattern of the other process (the screen-printing process). The laser then performs the patterning by connecting the corrected grid points with each other in the desired direction. We perform the associated high-precision measurement of the patterns' coordinates by using either a high-precision offline coordinate measuring machine or a high-resolution inline camera system with subsequent computer-based data processing. The latter inline method enables high throughput and is, in turn, of great interest for mass production of solar cells. In this paper, we demonstrate the alignment procedure approaches on "pPassDop" solar cells by adjusting a locally applied laser process to the directly following screen-printing step. This proof of principle includes both above-mentioned methods for coordinate determination in separate cell batches. Our innovative alignment procedures so far demonstrated the successful matching of 40-mu m-wide screen-printed contact fingers to 70-mu m-wide laser-processed lines over the entire area of 6-inch solar cells.

We investigate stacks of aluminum oxide (Al2O3) and boron-doped silicon nitride (SiNX:B) layers for the rear side passivation and local doping of p-type silicon solar cell samples aiming for the realization of bifacial passivated emitter and rear locally diffused (biPERL) solar cells. The local p(+)-doped back surface field regions are formed by laser doping and are electrically contacted using commercially available screen-printed and fired silver-aluminum (AgAl) or silver (Ag) contacts. This approach is referred to as "pPassDop". Laser doping results in highly-doped silicon with sheet resistances as low as 15 Omega/sq and surface doping concentrations up to 6x10(19) cm(-3). Low specific contact resistances around 1 m Omega cm(2) and 5 m Omega cm(2) are measured for the screen-printed and fired AgAl and Ag contacts, respectively. In addition, the influence of each individual layer within the pPassDop layer stack on the doping properties is investigated. In order to separate the impact of aluminum and boron doping, firstly the influence of the Al2O3 layer thickness (0 nm, 4 nm, 6 nm) below the SiNX:B capping layer is studied. Secondly, a conventional undoped SiNX capping layer is applied on a 6 nm-thick Al2O3 layer. The roles of each dopant are studied by measuring the doping profile and contact resistivity.

One major loss mechanism for currently relevant solar cells [e.g., passivated emitter and rear cells (PERC)] is locally enhanced recombination at the interface between semiconductor and metalization. For investigating these losses in detail, a reliable detection technique is crucial. The photovoltaics community frequently applies an area-weighted model to extract the local dark saturation current density in the metalized area of the emitter j(0,met)-e.g., from photoluminescence imaging (PLI) data. However, this model does not account for the nonuniformity of the excess carrier density Delta n within the sample during the measurement. Therefore, we compare numerical PLI simulations using Quokka3 to PLI measurements, to quantitatively reveal the impact of the nonuniformity of Delta n. Test structures with locally laser-ablated passivation on one side-serving as ideal test structures-are used for the experimental verification. Additionally, we show results on metalized samples. We find that the results (j(0,met)*) using the simulative approach for the laser-ablated samples exceed the results using the conventional area-weighted approach by more than 20%. For the metalized samples, we see a similar trend where the area-weighted approach again underestimates the results (j(0,met)), in this case by up to 20%. Based on our investigations, we show that the deviation between the results arises due to the assumption of uniform Delta n applied by the area-weighted approach.

Three approaches to close the efficiency gap between screen-printed Ag-paste and Ni/Cu/Ag-plated front contact metallization on industrial passivated emitter and rear cells (PERC) solar cells are presented in this paper. In the first approach, the POCl3 diffusion is adjusted to an emitter profile (reduced peak doping) for plated contacts. The second approach is to adapt the laser over doping (LOD) process of the phosphor silicate glass (PSG) to create the selective emitter to the properties of laser patterning and plating. In the third approach, we vary the process step order of front laser patterning and back side aluminum firing. The three approaches show very promising results. Efficiencies higher than 22% and open-circuit voltage V-OC values of close to 680 mV are reached on a cell area of 251,99cm(2). Overall the values excel the reference values obtained with screen printing and firing of Ag-paste. For future developments, V-OC values between 685 mV and 690 mV and efficiencies around 22.5% seem very likely.

We evaluate in detail the impact of sample properties on the accuracy of a simple area-weighted approach to determine the local dark saturation current density at metal contacts j(0,met). Using metallized samples, we further compare the apparent j(0,met) resulting from this simple approach to the j(0,met) value determined using numerical simulations with Quokka3. The analysis shows that the assumption of a uniform carrier density of the area-weighted approach leads to a significant underestimation of j(0,met) which depends strongly on sample properties as e.g. the base resistivity rho(B) and j(0,met) itself. This is confirmed by experimental data using conventional metallized samples, which demonstrate an underestimation of j(0,met) of up to 20% when using the area-weighted approach compared to numerical simulations.

We apply stacks of silicon nitride (SiNX) layers consisting of phosphorus-doped SiNX (SiNX: P) and undoped SiNX as first and second layer, on the front surface of p-type monocrystalline silicon passivated emitter and rear cells (PERC). The stack is deposited using plasma-enhanced chemical vapor deposition. These layer stacks provide excellent surface passivation and anti-reflection properties comparable to single undoped SiNX layers. A laser processing locally introduces the phosphorus dopants from the SiNX: P/SiNX layer stack into the silicon and locally removes the layer stack. Thereby, local highly doped areas with sheet resistances as low as R-sh approximate to 20 Omega/sq are obtained. We refer to this as the nPassDop approach, which provides local high doping and structuring of the anti-reflection coating in one process. The saturation current density of the laser processed areas is estimated to be as low as j(0, laser) approximate to 500 fA/cm (2). First implementations for large-area p-type Cz-Si PERC cells with self-aligned NiCuAg plated front contacts yield an energy conversion efficiency of 19.6%.

We demonstrate a bifaciality of 88.0% for 6-inch bifacial p-type Cz-Si passivated emitter and rear cells (biPERC) and increase their rear side energy conversion efficiency to 18.0% by minor adaptions in the fabrication sequence. We utilize the "pPassDop" concept on the cells' rear side that applies an aluminum oxide and a boron-doped silicon nitride (SiNX: B) layer stack for simultaneous passivation and doping source. Laser doping forms the local p-doped back surface field regions for these biPERL solar cells. Screen-printed silver-aluminum metallization contacts these regions. We also demonstrate the compatibility of the laser doping approach with conventional (undoped) SiNX capping layer to fabricate biPERL devices with screen-printed contacts.

The concept of attaching a second deposition step at the end of boron tribromide (BBr3) diffusion is introduced, where second deposition describes an active ni­tro­gen flow through the BBr3 bubbler. This approach pro­vides a high­er boron dose in the borosilicate glass (BSG) which facilitates the formation of laser‐doped se­lec­tive emitters. It is found that the second deposition hardly impacts the as‐diffused charge carrier con­cen­tra­tion profile in comparison to BBr3 dif­fu­sion without second deposition. The emitter sheet re­sis­tance Rsh ≈ 110 Ω sq−1 and emitter dark saturation cur­rent den­si­ty j0e ≈ 25 fA cm−2 (alkaline textured, Al2O3/SiNX passi­va­tion) are similar for both processes. The BBr3 diffusion process forms a BSG/silicon dioxide (SiO2) stack layer on the silicon. The BBr3 diffusion with second deposition step results in an 8 nm thicker BSG/SiO2 stack layer (total thickness: 42 nm) with fac­tor two higher boron dose compared to the BBr3 diffusion without second deposition. After laser doping, the charge carrier concentration is higher for the BBr3 pro­cess with second deposition resulting in stronger local doping with about 10 Ω sq−1 lower Rsh. For laser‐doped and Al2O3/SiNX‐passivated areas, a promising process combination results in j0e = (250 ± 30) fA cm−2 at Rsh = (65 ± 1) Ω sq−1.

The authors discuss industry related approaches at Fraunhofer ISE for bifacial p-type silicon solar cells, taking into account the well-known “passivated emitter and rear cell” (PERC), “passivated emitter and rear totally diffused” (PERT) and “passivated emitter and locally diffused” (PERL) architectures. In the case of PERC, challenges in terms of alignment, printability and the importance of bifaciality are addressed. In the case of PERT, a co-diffusion process is utilized to form the emitter and the back surface field simultaneously avoiding also critical shunts that can arise at the edges of such devices. For the PERL technology, the industrial feasible pPassDop approach is discussed. We report on front side energy conversion efficiencies for PERC of 21.4%, PERT of 20.5%, and PERL of 19.8%. Furthermore, bifaciality factors for PERC of 0.7, for PERT of 0.86, and for PERL of 0.89 are presented.