Three-terminal tandem solar cells enabled by back-contacted bottom cells featuring passivating, carrier-selective polysilicon based junctions

This thesis investigates back-contacted (IBC) bottom solar cells with passivating and carrier-selective POLO contacts with three terminals (3T-POLO-IBC cell). Such cells form the foundation of monolithic three-terminal tandem solar cells. This novel tandem solar cell enables the use of sub-cells with mismatched photocurrents. Thus, this tandem solar cell technology platform offers the flexibility with respect to subcell material selection, the ease of fabrication, and a robustness to spectral variations of incident light over the course of the day and year. Three building blocks of the 3T POLO IBC bottom solar cell, which are based on each other, are examined: First, the passivating and carrier-selective POLO contact. Second, the integration of POLO contacts on the rear side of a solar cell. Third, the principle of operation of a bottom cell with three terminals. In the first part, the process of charge carrier extraction at selective contacts to the photoabsorber is theoretically explored. The selectivity of a contact is defined on the basis of (reaction) kinetic considerations at the contact in terms of the rate ratio of desired processes to undesired processes. The extraction efficiency of charge carriers at the contact is derived as the ratio of the external voltage versus the internal voltage from a thermodynamic point of view. To emphasize the unifying nature of the definitions in this thesis, the existing literature definitions are calculated from the definitions in this thesis. The extraction efficiency is related to the selectivity coefficient of the contact and the limiting efficiency of a silicon solar cell with given contact selectivity is calculated accordingly. After the detailed theoretical investigation on selectivity, the properties of n+ and p+ POLO contacts are examined. Low saturation current densities between 2 fA/cm² and 18 fA/cm² and contact resistivities between 0.4mOhmcm² and 10mOhmcm² are found at the same time. It is shown that the efficient carrier transport of majority carriers is ensured by pinholes in the interfacial oxide. The resulting logarithmic selectivity coefficient of POLO contacts is determined to be above 15, which is one of the highest values measured. This makes POLO contacts predestined for solar cells with the highest efficiencies. POLO contacts are integrated on the rear side of a back-contact cell with POLO contacts for both polarities. Thereby, the p+ and n+ doped poly-Si on the backside of the solar cell form a parasitic graded p+n+ junction within the defect-rich poly-Si with a carrier lifetime of a few picoseconds. The arising recombination limits the achievable efficiency of the POLO-IBC cell to about 18%. For this reason, the parasitic junction is removed during the cell fabrication process by wet-chemically introducing a trench between the n+- and p+-doped poly-Si regions. The POLO-IBC cell with isolated n+- and p+ poly-Si regions achieves a certified efficiency of 24.25%. For the last part, a third POLO contact is added to the POLO-IBC cell and the 3T-IBC bottom cell is studied in detail using current-voltage measurements. First, the different realization options for a 3T tandem solar are sorted and the corresponding nomenclature is presented. Two different 3T IBC bottom cell architectures are identified. The first one – the unijunction bottom solar cell – contains a single minority carrier contact and two majority carrier contacts. The second one – the bipolar junction bottom solar cell – on the other hand, has two minority carrier contacts and a single majority carrier contact. Both 3T bottom cell architectures are fabricated based on a modified POLO-IBC fabrication process. The principles of operation and loss mechanisms are elucidated using J-V measurements on illuminated devices and by means of analytical modeling. The experiments show that the third contact of a 3T unijunction and bipolar junction bottom cell allows the collection or injection of additional minority or majority carriers from or into the bottom cell. Ideally, the power output of such a 3T bottom cell is nearly independent of the current density applied by the top cell. Therefore, no current matching of both subcells is required. However, the transport of majority carriers or minority carriers through the unijunction or bipolar junction bottom cell causes a loss, which, however, can be made negligible by a specific design of the bottom cell. The design rules are explained in detail. After the detailed investigations, a 3T unijunction bottom cell with a textured n+-POLO front contact with an efficiency of 20.3% and a simplified screen-printed PERC-like 3T bipolar junction bottom cell with 14.4% are developed. The latter is an attractive approach to utilize the dominant PERC technology in a low-cost tandem solar cell with maximum energy yield. Finally, the first 3T GaInP//POLO-IBC tandem cell demonstrator is fabricated with an efficiency of 27.3% and a net efficiency gain of 0.9% is demonstrated compared to the 2T operation of the 3T tandem cell.Die vorliegende Arbeit untersucht Rückkontakt-Bottomsolarzellen mit passivierenden und ladungsträger-selektiven POLO-Kontakten mit drei Anschlüssen (3T-POLO-IBC-Bottomzelle). Sie bilden das Fundament monolithischer Tandemsolarzellen mit drei Anschlüssen. Diese neuartigen Tandemsolarzelle erlaubt die Verwendung von Subzellen, dessen Fotoströme fehlangepasst sind. Damit bietet diese Tandemsolarzellen-Technologie Flexibilität bei der Materialauswahl der Subzellen, einfache Herstellbarkeit und Robustheit gegenüber spektraler Änderung des einfallenden Lichts im Tages- und Jahresverlauf. Es werden drei aufeinander aufbauende Bausteine der 3T-POLO-IBC-Bottomsolarzelle untersucht: Erstens, der passivierende und ladungsträger-selektive POLO-Kontakt. Zweitens, die Integration von POLO-Kontakten auf der Rückseite der Solarzelle. Drittens, die Funktionsweise einer Bottomzelle mit drei Anschlüssen. Im ersten Teil wird der Prozess der Ladungsträgerextraktion an selektiven Kontakten zum Fotoabsorber theoretisch ergründet. Die Selektivität eines Kontaktes wird auf der Grundlage von (reaktions-) kinetischen Betrachtungen am Kontakt als das Ratenverhältnis gewollter Prozesse zu ungewollten Prozessen definiert. Die Extraktionseffizienz von Ladungsträgern am Kontakt wird als das Verhältnis der externen Spannung gegenüber der internen Spannung aus thermodynamischen Gesichtspunkten abgeleitet. Um den vereinheitlichenden Charakter der Definitionen in dieser Arbeit hervorzuheben, werden die bestehenden Literatur-Definitionen aus den Definitionen in dieser Arbeit berechnet. Die Selektivität und Extraktionseffizienz werden miteinander korreliert und daraus der Wirkungsgrad einer Solarzelle mit vorgegebener Kontaktselektivität errechnet. Nach der detaillierten theoretischen Untersuchung der Selektivität werden die Eigenschaften von n+ und p+ POLO-Kontakten untersucht. Es werden niedrige Sättigungsstromdichten zwischen 2 fA/cm² und 18 fA/cm² und gleichzeitig Kontaktwiderstände zwischen 0,4 mOhmcm² und 10 mOhmcm² ermittelt. Es wird gezeigt, dass der effiziente Ladungsträgertransport der Majoritäten durch Pinholes im Grenzflächenoxid sichergestellt wird. Der resultierende logarithmische Selektivitäts-Koeffizient von POLO-Kontakten wird auf über 15 bestimmt. Damit gehören POLO-Kontakte zu den Kontakten mit der höchsten Selektivität und sind für Solarzellen mit höchsten Effizienzen prädestiniert. Die POLO-Kontakte werden auf der Rückseite einer Rückkontaktzelle mit POLO-Kontakten für beide Polaritäten integriert. Dabei formt das p+ und n+ dotierte Poly-Si auf der Rückseite der Solarzelle einen parasitären, gradierten p+n+-Übergang im defektreichen Poly-Si mit einer Ladungsträgerlebensdauer von wenigen Pikosekunden. Die resultierende Rekombination limitiert die erreichbare Effizienz der POLO-IBC-Zelle auf etwa 18%. Aus diesem Grund wird der parasitäre Übergang während des Zellherstellungsprozesses entfernt, indem ein Graben zwischen die n+- und p+-dotierten Poly-Si-Regionen nasschemisch eingebracht wird. Die POLO-IBC-Zelle mit isolierten n+- und p+-Poly-Si-Gebieten erzielt einen zertifizierten Wirkungsgrad von 24,25%. Für den letzten Baustein wird die POLO-IBC-Zelle um einen dritten POLO-Kontakt ergänzt und die 3T-IBC-Bottomzelle mittels Strom-Spannungsmessungen im Detail untersucht. Zuerst werden die unterschiedlichen Realisierungsmöglichkeiten für eine 3T-Tandemsolar einsortiert und die dazugehörige Nomenklatur vorgestellt. Dabei werden zwei verschiedene 3T-IBC-Bottomzellen-Architekturen unterschieden. Eine Unijunction-Bottomsolarzelle enthält einen einzigen Minoritätsladungsträgerkontakt und zwei Majoritätsträgerkontakte. Eine Bipolar-Junction-Bottomsolarzelle hingegen hat zwei Minoritätsladungsträgerkontakte und einen einzigen Majoritätsladungsträgerkontakt. Beide 3T-Bottomzell-Architekturen werden auf Basis eines modifizierten Herstellungsprozesses für POLO-IBC-Solarzellen realisiert. Das Funktionsprinzip und die Verlustmechanismen werden mit Hilfe von J-V -Messungen an beleuchteten Bauelementen und mit Hilfe analytischer Modellierung untersucht. Die Experimente zeigen, dass der dritte Kontakt einer 3T-Unijunction- und Bipolar-Junction-Bottomzelle das Sammeln oder Injizieren von zusätzlichen Minoritäts- oder Majoritätsladungsträgern aus der oder in die Bottomzelle ermöglicht. Im Idealfall ist die Leistungsabgabe einer solchen 3T-Bottomzelle nahezu unabhängig von der Stromdichte, die von der Topzelle angelegt wird. Daher ist keine Stromanpassung beider Subzellen erforderlich. Allerdings verursacht der Transport von Majoritätsladungsträgern bzw. Minoritätsladungsträgern durch die Unijunction- bzw. Bipolar-Junction-Bottomzelle hindurch einen Verlust, welcher jedoch durch eine gezielte Auslegung der Bottomzelle vernachlässigbar klein ausfallen kann. Die Auslegung wird im Detail erläutert. Schließlich wird eine 3T-Unijunction-Bottomzelle mit einem texturierten n+-POLO-Frontkontakt mit einem Wirkungsgrad von 20,3% und eine vereinfachte siebgedruckte PERC-ähnliche 3T-Bipolar-Junction-Bottomzelle mit 14,4% entwickelt. Letztere ist ein attraktiver Ansatz, um die dominierende PERC-Technologie in einer kostengünstigen Tandemsolarzelle mit maximaler Energieausbeute zu nutzen. Abschließend wird der erste 3T-GaInP//POLO-IBC-Tandemzellen-Demonstrator mit einem Wirkungsgrad von 27,3% hergestellt und ein Netto-Wirkungsgradgewinn von 0,9% im Vergleich zum 2T-Betrieb der 3T-Tandemzelle demonstriert

Rear side dielectrics on interdigitating p+-(i)-n+ back-contact solar cells − hydrogenation vs. charge effects

Polysilicon-on-oxide (POLO) passivating contacts and interdigitated back-contact (IBC) cell technologies have recently attracted a lot of interest as candidates for the implementation in the next generation of solar cells. An IBC cell with POLO junctions for both polarities-a POLO2-IBC cell-has to electrically isolate the highly defective p+ and n+ poly-Si regions on the rear side of the cell to avoid parasitic recombination. Inserting an initially undoped, intrinsic (i) region between the p+ and n+ poly-Si regions was demonstrated to successfully prevent the parasitic recombination in the transition region of ISFH's 26.1%-efficient POLO2-IBC cell. In order to further improve the conversion efficiency towards 27%, we apply hydrogen-donating dielectric layer stacks to the p+-(i)-n+ POLO interdigitating rear side to enhance the passivation quality of the POLO junctions. We indeed show a significant improvement of POLO junctions on symmetrical full-Area homogenously doped reference samples, but when we apply a hydrogen-donating layer stack on the p+-(i)-n+ POLO interdigitating rear side, we observe a strong degradation in the performance of the POLO2-IBC cell. We attribute this to the formation of a conductive channel between the p+ and n+ poly-Si regions due to the strong negative charge density of the hydrogen-donating layer stack

Recombination Behavior of Photolithography-free Back Junction Back Contact Solar Cells with Carrier-selective Polysilicon on Oxide Junctions for Both Polarities

We report on ion-implanted, inkjet patterned back junction back contact silicon solar cells with POLysilicon on Oxide (POLO) junctions for both polarities – n+ doped BSF and p+ doped emitter. The recombination behavior is investigated at two different processing stages: before and after trench separation of p+ and n+ regions within polysilicon (poly-Si). Before trench separation, we find a systematic dependence of the recombination behavior on the BSF index, i.e. the p+n+-junction meander length in the poly-Si. Obviously, recombination at the p+n+-junction in the poly-Si limits the implied open circuit voltage Voc,impl. at one sun illumination and the implied pseudo fill factor pFFimpl. to 695 mV and 80%, respectively. After trench isolation, however, Voc,impl (pFFimpl.) values increase up to 730 mV (85.5%), corresponding to a pseudo-efficiency of 26.2% for an assumed short circuit current density Jsc of 42 mA/cm2. We demonstrate a photolithography-free back junction back contacted solar cell with p-type and n-type POLO junctions with an in-house measured champion efficiency of 23.9% on a designated area of 3.97 cm2. This efficiency is mainly limited by the imperfect passivation in the undoped trench regions and at the undoped front side.EU/FP7/60849

Firing stability of tube furnace-annealed n-type poly-Si on oxide junctions

Stability of the passivation quality of poly-Si on oxide junctions against the conventional mainstream high-temperature screen-print firing processes is highly desirable and also expected since the poly-Si on oxide preparation occurs at higher temperatures and for longer durations than firing. We measure recombination current densities (J0) and interface state densities (Dit) of symmetrical samples with n-type poly-Si contacts before and after firing. Samples without a capping dielectric layer show a significant deterioration of the passivation quality during firing. The Dit values are (3 ± 0.2) x 1011 and (8 ± 2) x 1011 eV/cm2 when fired at 620°C and 900°C, respectively. The activation energy in an Arrhenius fit of Dit versus the firing temperature is 0.30 ± 0.03 eV. This indicates that thermally induced desorption of hydrogen from Si-H bonds at the poly-Si/SiOx interface is not the root cause of depassivation. Postfiring annealing at 425°C can improve the passivation again. Samples with SiNx capping layers show an increase in J0 up to about 100 fA/cm2 by firing, which can be attributed to blistering and is not reversed by annealing at 425°C. On the other hand, blistering does not occur in poly-Si samples capped with AlOx layers or AlOx/SiNy stacks, and J0 values of 2–5 fA/cm2 can be achieved after firing. Those findings suggest that a combination of two effects might be the root cause of the increase in J0 and Dit: thermal stress at the SiOz interface during firing and blistering. Blistering is presumed to occur when the hydrogen concentration in the capping layers exceeds a certain level

Characterization of multiterminal tandem photovoltaic devices and their subcell coupling

Three-terminal (3T) and four-terminal (4T) tandem photovoltaic (PV) devices using various materials have been increasingly reported in the literature, but measurement standards are lacking. Here, multiterminal devices measured as functions of two load variables are characterized unambiguously as functions of three device voltages or currents on hexagonal plots. We demonstrate these measurement techniques using two GaInP/GaAs tandem solar cells, with a middle contact between the two subcells, as example 3T devices with both series-connected and reverse-connected subcells. Coupling mechanisms between the subcells are quantified within the context of a simple equivalent optoelectronic circuit. Electrical and optical coupling mechanisms are most clearly revealed using coupled dark measurements. These measurements are sensitive enough to observe very small luminescent coupling from the bottom subcell to the top subcell in the prototype 3T device. Quick simplified measurement techniques are also discussed within the context of the complete characterization

2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four‐Terminal Tandems with Silicon and CIGS

Wide-bandgap perovskite solar cells (PSCs) with optimal bandgap (E$_{g}$) and high power conversion efficiency (PCE) are key to high-performance perovskite-based tandem photovoltaics. A 2D/3D perovskite heterostructure passivation is employed for double-cation wide-bandgap PSCs with engineered bandgap (1.65 eV ≤ E$_{g}$ ≤ 1.85 eV), which results in improved stabilized PCEs and a strong enhancement in open-circuit voltages of around 45 mV compared to reference devices for all investigated bandgaps. Making use of this strategy, semitransparent PSCs with engineered bandgap are developed, which show stabilized PCEs of up to 25.7% and 25.0% in fourterminal perovskite/c-Si and perovskite/CIGS tandem solar cells, respectively. Moreover, comparable tandem PCEs are observed for a broad range of perovskite bandgaps. For the first time, the robustness of the four-terminal tandem configuration with respect to variations in the perovskite bandgap for two state-of-the-art bottom solar cells is experimentally validated

Nanostructured front electrodes for perovskite/c-Si tandem photovoltaics

The rise in the power conversion efficiency (PCE) of perovskite solar cells has triggered enormous interest in perovskite-based tandem photovoltaics. One key challenge is to achieve high transmission of low energy photons into the bottom cell. Here, nanostructured front electrodes for 4-terminal perovskite/crystalline-silicon (perovskite/c-Si) tandem solar cells are developed by conformal deposition of indium tin oxide (ITO) on self-assembled polystyrene nanopillars. The nanostructured ITO is optimized for reduced reflection and increased transmission with a tradeoff in increased sheet resistance. In the optimum case, the nanostructured ITO electrodes enhance the transmittance by ∼7% (relative) compared to planar references. Perovskite/c-Si tandem devices with nanostructured ITO exhibit enhanced short-circuit current density (2.9 mA/cm2 absolute) and PCE (1.7% absolute) in the bottom c-Si solar cell compared to the reference. The improved light in-coupling is more pronounced for elevated angle of incidence. Energy yield enhancement up to ∼10% (relative) is achieved for perovskite/c-Si tandem architecture with the nanostructured ITO electrodes. It is also shown that these nanostructured ITO electrodes are also compatible with various other perovskite-based tandem architectures and bear the potential to improve the PCE up to 27.0%

Triple-junction perovskite-perovskite-silicon solar cells with power conversion efficiency of 24.4%

The recent tremendous progress in monolithic perovskite-based double-junction solar cells is just the start of a new era of ultra-high-efficiency multi-junction photovoltaics. We report on triple-junction perovskite-perovskite-silicon solar cells with a record power conversion efficiency of 24.4%. Optimizing the light management of each perovskite sub-cell (∼1.84 and ∼1.52 eV for top and middle cells, respectively), we maximize the current generation up to 11.6 mA cm−2. Key to this achievement was our development of a high-performance middle perovskite sub-cell, employing a stable pure-α-phase high-quality formamidinium lead iodide perovskite thin film (free of wrinkles, cracks, and pinholes). This enables a high open-circuit voltage of 2.84 V in a triple junction. Non-encapsulated triple-junction devices retain up to 96.6% of their initial efficiency if stored in the dark at 85 °C for 1081 h

Rear side dielectrics on interdigitating

Polysilicon-on-oxide (POLO) passivating contacts and interdigitated back-contact (IBC) cell technologies have recently attracted a lot of interest as candidates for the implementation in the next generation of solar cells. An IBC cell with POLO junctions for both polarities − a POLO2-IBC cell − has to electrically isolate the highly defective p+ and n+ poly-Si regions on the rear side of the cell to avoid parasitic recombination. Inserting an initially undoped, intrinsic (i) region between the p+ and n+ poly-Si regions was demonstrated to successfully prevent the parasitic recombination in the transition region of ISFH's 26.1%-efficient POLO2-IBC cell. In order to further improve the conversion efficiency towards 27%, we apply hydrogen-donating dielectric layer stacks to the p+-(i)-n+ POLO interdigitating rear side to enhance the passivation quality of the POLO junctions. We indeed show a significant improvement of POLO junctions on symmetrical full-area homogenously doped reference samples, but when we apply a hydrogen-donating layer stack on the p+-(i)-n+ POLO interdigitating rear side, we observe a strong degradation in the performance of the POLO2-IBC cell. We attribute this to the formation of a conductive channel between the p+ and n+ poly-Si regions due to the strong negative charge density of the hydrogen-donating layer stack