Sushobhan Avasthi

Role of Majority and Minority Carrier Barriers Silicon/ Organic Hybrid Heterojunction Solar Cells

A hybrid approach to solar cells is demonstrated in which a silicon p-n junction, used in conventional silicon-based photovoltaics, is replaced by a room-temperature fabricated silicon/organic heterojunction. The unique advantage of silicon/organic heterojunction is that it exploits the cost advantage of organic semiconductors and the performance advantages of silicon to enable potentially low-cost, efficient solar cells.

Hole-blocking titanium-oxide/silicon heterojunction and its application to photovoltaics

In contrast to the numerous reports on narrow-bandgap heterojunctions on silicon, such as strained Si1−xGex on silicon, there have been very few accounts of wide-bandgap semiconducting heterojunctions on silicon. Here, we present a wide-bandgap heterojunction—between titanium oxide and crystalline silicon—where the titanium oxide is deposited via a metal-organic chemical vapor deposition process at substrate temperatures of only 80–100 °C. The deposited films are conformal and smooth at the nanometer scale. Electrically, the TiO2/Si heterojunction prevents transport of holes while allowing transport of electrons. This selective carrier blocking is used to demonstrate a low-temperature processed silicon solar cell.

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.

Silicon surface passivation by an organic overlayer of 9,10-phenanthrenequinone

Merged organic-silicon heterojunction devices require the passivation of dangling bonds at the silicon surface, preferably with a low-temperature process. In this paper, we demonstrate the high-quality passivation of the silicon (100) surface using an organic molecule (9,10-phenanthrenequinone, PQ). PQ reacts with the dangling bonds, thus providing a bridge between organic semiconductors and silicon. We measure low recombination velocities (∼150 cm/s) at the PQ-silicon interface. Metal/organic-insulator/silicon capacitors and transistors prove that at PQ-silicon interface, the Fermi level can be modulated. The formation of an inversion layer with electron mobility of 600 cm2/V∙s further demonstrates the passivation quality of PQ.

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%.

Hole-blocking TiO2/silicon heterojunction for silicon photovoltaics

Narrow bandgap heterojunctions on crystalline silicon such as Si/Si1-xGex are now in widespread use, but to date there has been little progress on widegap heterojunctions on silicon. In this abstract, we report: (i) TiO2/Si heterojunction with a band alignment which blocks holes from silicon but freely passes electrons, and (ii) the application of this heterojunction to form a photovoltaic cell on silicon with no p-n junction, and all fabrication below a temperature of 75 °C.

Charge separation and minority carrier injection in P3HT-silicon heterojunction solar cells

In this work we investigate the behavior of carrier absorption and minority carrier injection in heterojunction solar cells fabricated by spin-coating the organic semiconductor poly(3-hexylthiophene) (P3HT) on n-type crystalline silicon. Using this structure we recently demonstrated a device with open-circuit voltage (VOC) of 0.59 V and short-circuit currents (ISC) of 22 mA/cm2 at AM 1.5 conditions [1–2]. In this paper we show, using capacitance-voltage characteristics, that there is a large depletion region in silicon which is responsible for the separation of photogenerated carriers. Furthermore, by measuring minority carrier storage times, we show that the dominant forward-bias dark-current component in these devices is the injection of minority carriers from the anode, through P3HT, in to silicon. This confirms that P3HT functions as a p-type heterojunction contact to silicon that blocks electrons but not holes, explaining the high VOC we observe.