Silicon heterojunction (HJ) solar cells with different rear passivation and contact designs were fabricated on ∼25 μm semiconductor-on-metal (SOM) exfoliated substrates. It was found that the performance of these cells is limited by recombination at the rear-surface. Employing the dual-HJ architecture resulted in the improvement of open-circuit voltage (Voc) from 605 mV (single-HJ) to 645 mV with no front side intrinsic amorphous silicon (i-layer) passivation. Addition of un-optimized front side i-layer passivation resulted in further enhancement in Voc to 662 mV. Pathways to achieving further improvement in the performance of HJ solar cells on ultra-thin SOM substrates are discussed.

A thin, flexible monocrystalline germanium (c-Ge) heterojunction solar cell has been developed based on a novel kerfless exfoliation process and remote plasma-enhanced chemical vapor deposition (RPCVD) of hydrogenated amorphous silicon (a-Si:H). The performance of the exfoliated 50μm thick and bulk 500μm Ge heterojunction cells is compared in this paper. A superior conversion efficiency of 5.28% was achieved with the 50μm exfoliated Ge cell versus 1.78% for the bulk Ge cell, in agreement with simulation results. A record fill factor of 58.1% for an a-Si:H/c-Ge heterojunction cell was obtained with the exfoliated cell. Moreover, the conversion efficiency achieved with the 50μm exfoliated cell (without intrinsic a-Si:H passivation) is comparable to the best reported in literature with bulk Ge heterojunction cells and intrinsic a-S:H passivation.

For the first time, a remote plasma chemical vapor deposition (RPCVD) based c-Si/a-Si heterojunction solar cell process was developed on thin crystalline silicon semiconductor-on-metal (SOM) substrate. In RPCVD systems, deposition temperature, deposition rate, and the distance of the sample from the plasma source can be varied to minimize the surface damage and enhance passivation quality. A silicon heterojunction (HJ) cell without intrinsic a-Si layer passivation was fabricated on an exfoliated ∼25µm c-Si SOM foil, with an efficiency of 13.4% and open-circuit voltage of 645mV. Losses in these devices were analyzed by numerical simulations and optimum device structure was designed and performance predicted.

In this work we propose and demonstrate a novel and cost-effective method to fabricate bifacial cells with conventional homojunction architecture. The method combines benefits of lithography-less, self-aligned patterning during deposition of antireflective coating (ARC) and simultaneous metallization of both surfaces aided by electroplating. We have fabricated a conventional diffused n+pp+ junction bifacial solar cell on a monocrystalline silicon (c-Si) substrate using this method. Electrochemically grown nickel is used to simultaneously form front and back electrodes. The bifacial solar cell fabricated with an un-optimized process has a front and rear efficiencies (under AM1.5G one sun illumination) of 12% and 8.66%, respectively. Part of the low performance of the cell is attributed to poor quality of the passivation layer and the post deposition annealing to reduce pinholes in deposited SiNx layer to prevent parasitic plating.

In this work we propose and demonstrate a novel method to fabricate semiconductor devices using a shadow mask that allows selective deposition of PECVD layers and selective exposure of doped silicon regions. The exposed regions then act as seed layers for selective metallization by a process such as electroplating. A device structure taking advantage of this process can be completely done without any photolithographic steps. In semiconductor processes where submicron-feature sizes are generally not required and reducing cost of manufacturing is key (e.g. in solar cells), this proposed process flow could be really useful. We have fabricated a conventional diffused p-n junction monofacial solar cell on a monocrystalline silicon (c-Si) substrate using this method. Nickel is electrochemically grown to form front surface electrode. The monofacial solar cell fabricated with an un-optimized process shows an efficiency of 14.5% under AM1.5G one sun illumination. The feature size of metal electrodes formed in this way is 80μm and could be narrowed down to smaller size-width. Optimization of doping, surface passivation and metallization would improve the efficiency even further to values in the high teens by improving open circuit voltage (VOC), current density (JSC) and Fill Factor (FF).

The crystalline Si photovoltaic industry has been scaling down the Si wafer thickness in order to reduce costs and potentially attain higher efficiencies by minimizing bulk recombination. However, cell manufacturers are struggling to reduce the wafer thickness below 150μm as there are no economically viable technologies for manufacturing very thin Si wafers and such thin silicon wafers impose stringent handling requirements as wafer breakage and yield loss impact final module cost. We have previously reported a novel kerfless exfoliation technology capable of producing ultra thin 25μm thin flexible mono c-Si foils from thick Si wafers. In this work, we report on scaling the technology to 8-inch diameter wafers. A 25μm thin exfoliated monocrystalline Si solar cell with a front heterojunction emitter and a diffused back surface field structure has been fabricated with a power conversion efficiency of 14.9%. Simulations show that with optimized texturing of the foil and better surface passivation, higher efficiencies (20%) can be attained. We have also fabricated dual heterojunction devices on 25μm thin exfoliated Si, which show high V oc of 680mV. Due to the kerfless exfoliation process and wafer reuse, a final cell cost of $0.30/Wp can be achieved.

A thin, flexible monocrystalline germanium (c-Ge) heterojunction solar cell has been developed based on a cost-effective kerfless exfoliation process and remote plasma-enhanced chemical vapor deposition (RPCVD) of hydrogenated amorphous silicon (a-Si:H). The performance of the exfoliated 50μm thick and bulk 500μm Ge heterojunction cells are compared in this paper. A superior conversion efficiency of 5.28% was achieved with the 50μm exfoliated Ge cell versus 1.78% for the bulk Ge cell, in agreement with simulation results. A record fill factor of 58.1% for an a-Si:H/c-Ge heterojunction cell is obtained with the exfoliated cell. Moreover, the conversion efficiency achieved with the 50μm exfoliated cell (without intrinsic a-Si:H passivation) is comparable to the best reported in literature with bulk Ge heterojunction cells and intrinsic a-S:H passivation.