Udo Romer

IBC solar cells have gained tremendous interest in the PV industry as next-generation technology. ISFH has developed a lean manufacturing process sequence for POLO IBC solar cells applying p-type Cz wafers, an Al-BSF base contact and local PECVD deposition of the SiO _x N _y /n-type polysilicon emitter through a glass shadow mask. In this paper, we report a new best POLO IBC cell efficiency of 24.5% processed at ISFH on M2 wafer size. In 2024, we started to transfer the POLO IBC process from the ISFH SolarTeC to the Kalyon PV manufacturing line using their M10 sized p-type Ga-doped Cz wafers and cell production tools. With Kalyon PV's wet chemistry and PECVD AlO _x /SiN tools good surface passivation is demonstrated by obtaining an iV _oc up to 727 mV using textured, rear side polished AlO _x /SiN passivated test wafers. Kalyon PV targets to process first M10-sized POLO IBC solar cells till end of 2025. However, the POLO IBC efficiency will be limited to below 25.5% by the carrier recombination at the Al-BSF base contact. To overcome this limitation aiming at n-type poly / p-type polysilicon POLO ² IBC cell efficiencies beyond 26%, EnPV and ISFH optimized a carrier selective SiO _x /p-type polysilicon layer stack yielding a new best median saturation current density J ₀ = 4 ± 1 fA/cm ² using industrial tools for the wet chemically grown SiO _x and the in-situ doped p-type polysilicon layer. ISFH is applying the SiO _x /p-type polysilicon layer stack to develop a novel industrial processing sequence for the POLO ² IBC solar cell. We deposit both polysilicon layers in-situ-doped full-area and laser-structure both polysilicon polarities. Using lab-type tools and p-type float zone (FZ) wafers, a small-area POLO ² IBC solar cell with 25.5% efficiency has been developed at ISFH. Using M2-sized n-type Cz wafers, a novel IBC trench patterning process, and solely industrial processing tools in the ISFH SolarTeC targeting very cost-effective processes for etch barrier formation, laser structuring, and polysilicon etching, we obtain a measured implied V _oc = 735 mV of POLO ² IBC cells processed without metal contacts. Since the polysilicon contacts minimize carrier recombination at metal contacts, the implied V _oc value demonstrates the high V _oc potential of this promising new POLO ² IBC manufacturing process thereby indicating a conversion efficiency potential above 26%.

Shading losses are a longstanding obstacle in photovoltaic devices, particularly in concentrator photovoltaics, where the tradeoff between shading and resistive losses limits the concentration at which the highest power conversion efficiency is achieved to values far below the capabilities of concentrator optics. Here, we demonstrate a simple and scalable fabrication method that enables large front metal coverage while keeping shading losses to a minimum. Soft-imprint lithography is used to create trenches in a transparent polymer above the metal contacts, enabling cloaking via refraction at a range of angles near normal incidence. Using optical characterization techniques, we first confirm that the metal contacts are indeed optically cloaked. We then demonstrate an increase in short-circuit current density from 29.95 to 39.12 mA/cm ² for a Si solar cell with 25% front metal coverage before and after patterning, respectively. We investigate the angular performance of the trench pattern and further demonstrate how such a cloaking strategy could be implemented in concentrator photovoltaics to enable efficiency peaks at concentrations beyond 1000 suns.

Concentrated photovoltaic (CPV) systems can achieve high energy conversion efficiencies due to their use of concentrating optics and highly efficient solar cells; however, power losses arising from the front surface reflection and metallization shading typically contribute a significant fraction to the total power loss for these devices. In flat silicon photovoltaic (PV) modules, the front‐metal finger shape is often shaped to minimize shading losses. Finger shape is even more relevant for CPV cells where the current density and front metal shading are much higher. This study compares the optical performance of five light management finger designs in CPV systems with two different refractive homogenizers. The optical efficiency and loss analysis were determined using ray‐tracing simulations. It is shown that shaped, light diverting metal fingers can improve the optical efficiency in CPV systems by up to 9.9% absolute compared to rectangular fingers, corresponding to an absolute cell efficiency increase of 3.0%. Although sharp edged triangular finger structures may be challenging to fabricate, significant optical gains can still be achieved with trapezoid or rounded cap finger shapes.

Historically, the performance of monolithically-integrated GaAsP/Si tandem solar cells has been limited by the presence of elevated TDD in the GaAsP subcell. However, our recent development of low-TDD GaAsP/Si virtual substrates has greatly improved the near-term potential for achieving high efficiency devices. In this work, development efforts employing this new low-TDD platform will be presented. First, our current certified record GaAsP/Si tandem cell (23.4% AM1.5G) is briefly analyzed, identifying the root cause of critical fill factor losses, which in turn informs subsequent design and optimization efforts. Second, the experimental development of high performance GaAsP subcells is shown, following device design strategies that improve J SC and fill factor without sacrificing V OC . Finally, conversion efficiency projections demonstrate that the achieved top cell performance metrics, combined with those expected from the newest bottom cell iteration, should enable ~27% efficient tandems.

Application of 266-nm picosecond (ps) laser ablation and copper (Cu)-plated metallization to p-type selective emitter (SE) passivated emitter and rear cells (PERC) is reported in this paper. Use of a 266-nm ps laser resulted in similar laser-induced periodic surface structures as observed for 355-nm ps laser ablation of a silicon (Si) nitride antireflection coating (ARC) on random-textured Si solar cell surfaces. In addition, it is shown that 266-nm ps laser ablation results in the formation of amorphous Si with an underlying distorted crystalline Si layer at the laser-ablated surfaces. The successful alignment of laser-ablated openings to the heavily doped SE regions resulted in a comparable cell efficiency of Cu-plated SE PERC cells to screen-printed controls, with a maximum cell efficiency of 20.6% being achieved for the Cu-plated cells. The plated cell performance was limited by the recombination losses, and in particular nonideal recombination caused by the use of a shallow emitter, which had been optimized for screen-printed metallization. Engineering of an SE with a junction depth of 0.52 μm in the heavily doped regions resulted in a 0.3% absolute increase in pseudo fill factor and demonstrated the importance of displacing the p-n junction from the laser-ablated Si surface. Although 355-nm ps laser ablation has been demonstrated to result in strong busbar adhesion in previous reports of Cu-plated cells, significant variability in the busbar adhesion of the fully plated SE PERC cells resulted by 266-nm ps laser ablation. The predicted increased sensitivity of 266-nm laser ablation to the ARC thickness and the possibility that surface oxides were not uniformly removed across wafers before plating may have affected the uniformity of silicide formation and hence the adhesion of the plated busbars.

Indium tungsten oxide (IWO) can provide advantages over indium tin oxide (ITO) as a transparent conducting oxide silicon heterojunction (SHJ) solar cells due to its lower absorption in the long wavelength region. However, low-temperature metallization methods are required for IWO-coated cells. This paper introduces a new process for the direct Cu plating to IWO surfaces for SHJ cells. The process, which employs a rapidly-plated initial seed Cu layer, can achieve strong interfacial adhesion and specific resistivity values as low as 1.81 ± 0.32 mΩ-cm ² . It is proposed that the enhanced contact adhesion resulted from the reduced Cu nuclei grain size at the IWO-Cu interface and increased surface roughness.