Ken A. Nagamatsu

A 12% Efficient Silicon/PEDOT:PSS Heterojunction Solar Cell Fabricated at < 100 °C

Solar cells based on a heterojunction between crystalline silicon and the organic polymer PEDOT:PSS were fabricated at temperatures <;100 °C by spin coating. The Si/PEDOT interface blocks electrons in n-type silicon from moving to the anode and functions as a low-temperature alternative to diffused p- n junctions. The device takes advantage of the light absorption and transport properties of silicon and combines it with the simplicity of fabrication afforded by organics. Reverse recovery measurements were used to analyze the electron-blocking effectiveness of the heterojunction. The data show that current in the device is primarily due to holes injected from the anode into the silicon. At AM1.5, Si/PEDOT heterojunction solar cells achieve power conversion efficiency of 11.7%, which is among the highest reported values for this class of devices.

Titanium dioxide/silicon hole-blocking selective contact to enable double-heterojunction crystalline silicon-based solar cell

In this work, we use an electron-selective titanium dioxide (TiO2) heterojunction contact to silicon to block minority carrier holes in the silicon from recombining at the cathode contact of a silicon-based photovoltaic device. We present four pieces of evidence demonstrating the beneficial effect of adding the TiO2 hole-blocking layer: reduced dark current, increased open circuit voltage (VOC), increased quantum efficiency at longer wavelengths, and increased stored minority carrier charge under forward bias. The importance of a low rate of recombination of minority carriers at the Si/TiO2 interface for effective blocking of minority carriers is quantitatively described. The anode is made of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) heterojunction to silicon which forms a hole selective contact, so that the entire device is made at a maximum temperature of 100 °C, with no doping gradients or junctions in the silicon. A low rate of recombination of minority carriers at the Si/TiO2 interface is crucial for effective blocking of minority carriers. Such a pair of complementary carrier-selective heterojunctions offers a path towards high-efficiency silicon solar cells using relatively simple and near-room temperature fabrication techniques.

Improved efficiency of hybrid organic photovoltaics by pulsed laser sintering of silver nanowire network transparent electrode.

In this Research Article, we demonstrate pulsed laser processing of a silver nanowire network transparent conductor on top of an otherwise complete solar cell. The macroscopic pulsed laser irradiation serves to sinter nanowire-nanowire junctions on the nanoscale, leading to a much more conductive electrode. We fabricate hybrid silicon/organic heterojunction photovoltaic devices, which have ITO-free, solution processed, and laser processed transparent electrodes. Furthermore, devices which have high resistive losses show up to a 35% increase in power conversion efficiency after laser processing. We perform this study over a range of laser fluences, and a range of nanowire area coverage to investigate the sintering mechanism of nanowires inside of a device stack. The increase in device performance is modeled using a simple photovoltaic diode approach and compares favorably to the experimental data.

Low-Temperature Synthesis of a TiO2/Si Heterojunction

The classical SiO2/Si interface, which is the basis of integrated circuit technology, is prepared by thermal oxidation followed by high temperature (>800 °C) annealing. Here we show that an interface synthesized between titanium dioxide (TiO2) and hydrogen-terminated silicon (H:Si) is a highly efficient solar cell heterojunction that can be prepared under typical laboratory conditions from a simple organometallic precursor. A thin film of TiO2 is grown on the surface of H:Si through a sequence of vapor deposition of titanium tetra(tert-butoxide) (1) and heating to 100 °C. The TiO2 film serves as a hole-blocking layer in a TiO2/Si heterojunction solar cell. Further heating to 250 °C and then treating with a dilute solution of 1 yields a hole surface recombination velocity of 16 cm/s, which is comparable to the best values reported for the classical SiO2/Si interface. The outstanding performance of this heterojunction is attributed to Si-O-Ti bonding at the TiO2/Si interface, which was probed by angle-resolved X-ray photoelectron spectroscopy. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed that Si-H bonds remain even after annealing at 250 °C. The ease and scalability of the synthetic route employed and the quality of the interface it provides suggest that this surface chemistry has the potential to enable fundamentally new, efficient silicon solar cell devices.

Double-heterojunction crystalline silicon solar cell fabricated at 250°C with 12.9 % efficiency

Double-heterojunction crystalline silicon solar cells were fabricated at temperatures of <;250°C using Si/organic and Si/metal-oxide heterojunctions, but no p-n junction in silicon. The first heterojunction, formed by spin-coating organic PEDOT:PSS on n-type silicon, functions as a front surface field that separates the photogenerated carriers and blocks electron dark-current while allowing hole photo-current to pass though. The second heterojunction, formed via metal-organic chemical vapor deposition of titanium dioxide on n-type silicon, functions as a back surface field that reduces hole dark-current while allowing electron photocurrent to pass through. Compared to a single heterojunction solar cell with only a Si/PEDOT heterojunction, the double-heterojunction device is more efficient with a power conversion efficiency of 12.9% under AM1.5.

Hole-blocking crystalline-silicon/titanium-oxide heterojunction with very low interface recombination velocity

We demonstrate a hole-blocking crystalline-silicon/titanium-oxide heterojunction that can be fabricated by a modified MOCVD process at only 100 oC substrate temperature. Ultra thin layers of only 1-4 nm TiO2 can be reliably deposited on silicon with no pinholes. Band alignment at the Si/TiO2, experimentally determined using surface spectroscopy, confirms that Si/TiO2 interface has a large barrier at the valence band, which blocks holes. The hole-blocking characteristics allow the Si/TiO2 heterojunction solar cells to achieve power conversion efficiencies > 7%. Finally, the electrical quality of the Si/TiO2 interface was characterized in terms of interface recombination velocity. We show that annealed Si/TiO2 interfaces can achieve recombination velocities of ~ 200 cm/s.

Stable low-recombination n-Si/TiO 2 hole-blocking interface and its effect on silicon heterojunction photovoltaics

TiO2 deposited on (100) crystalline silicon at near room temperature results in a hole-blocking, electron-transparent heterojunction. In this paper, we show that this interface can have a minority carrier recombination velocity on the order of 100 cm/s, which is stable for over 5 months in air. Second, we model the effect of such interfaces to replace the diffused n+/n (back surface field) layer at the cathode of p+/n and double heterojunction crystalline silicon solar cells. Simulations show that using TiO2/n-Si with the measured values of interface recombination velocity as a replacement for the n+/n diffusion at the cathode contact would yield power conversion efficiencies greater than 23%.

Double-heterojunction crystalline silicon solar cell with electron-selective TiO2 cathode contact fabricated at 100°C with open-circuit voltage of 640 mV

A double-heterojunction c-Si solar cell was fabricated at maximum process temperature of 100°C. We demonstrate an electron-selective passivated contact to Si using TiO2, which increased the open-circuit voltage by 45 mV compared to a device with a direct metal to n-type substrate contact. In the fabricated structure, PEDOT/Si replaced the front-side p-n junction of conventional Si-based solar cells while the Si/TiO2 interface is formed on the back-side. Compared to previous work [1], the Voc has increased from 620 to 640 mV while maintaining a maximum process temperature of 100°C. Critical to the improved performance is better passivation of the Si/TiO2 interface. The increase in Voc can be attributed to an interface recombination velocity of ~75 cm/s, which is consistent with photoconductance decay measurements.

Wide bandgap HBT on crystalline silicon using electron-blocking PEDOT:PSS emitter

Heterojunction bipolar transistors in silicon using a narrow band gap base (e.g. Si/SiGe/Si) are well known and have been in production for many years. However a wide bandgap emitter on silicon has proved elusive (Fig 1(a)). Recently it has been shown that heterojunctions between crystalline silicon and organic semiconductors such as PEDOT or P3HT can block electrons from entering the organic [1, 2]. In this work we used the Si/PEDOT heterojunction to demonstrate a HBT with an organic semiconductor wide bandgap emitter, and a crystalline silicon base.

Extraction of Front- and Rear-Interface Recombination in Silicon Double-Heterojunction Solar Cells by Reverse Bias Transients

We present a method, based upon reverse-recovery (RR) transient measurements, for determining the interface recombination parameters of double-sided heterojunction solar cells. A physics-based model is developed, and normalized parameters are used to provide results that can be scaled to arbitrary wafer thickness and minority-carrier diffusion coefficient. In the case of dominant recombination at only one interface, interface recombination velocity can be extracted directly from RR times. In devices with significant recombination at both interfaces, numerical modeling must be used. The effects of minority-carrier current spreading in small devices can be corrected for analytically. The results are then applied to both PEDOT/ n-Si and PEDOT/n-Si/TiO2 heterojunction cells. We find that the PEDOT/n-Si interface, despite favorable band offsets and a significant built-in voltage, is not an ideal hole injector because of recombination at the PEDOT/n-Si interface. We also find that the effective surface recombination velocity at the Si-TiO2 interface in a metallized device is 330 cm/s, confirming that the interface has a low defect density. Finally, we reflect on the significance of these results for the further development of silicon heterojunction cells.