Jeremy R. Poindexter

3.88% Efficient Tin Sulfide Solar Cells using Congruent Thermal Evaporation

Tin sulfide (SnS), as a promising absorber material in thin-film photovoltaic devices, is described. Here, it is confirmed that SnS evaporates congruently, which provides facile composition control akin to cadmium telluride. A SnS heterojunction solar cell is demons trated, which has a power conversion efficiency of 3.88% (certified), and an empirical loss analysis is presented to guide further performance improvements.

Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber.

Methylammonium lead halide (MAPbX3 ) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.

Framework to predict optimal buffer layer pairing for thin film solar cell absorbers: A case study for tin sulfide/zinc oxysulfide

An outstanding challenge in the development of novel functional materials for optoelectronic devices is identifying suitable charge-carrier contact layers. Herein, we simulate the photovoltaic device performance of various n-type contact material pairings with tin(II) sulfide (SnS), a p-type absorber. The performance of the contacting material, and resulting device efficiency, depend most strongly on two variables: conduction band offset between absorber and contact layer, and doping concentration within the contact layer. By generating a 2D contour plot of device efficiency as a function of these two variables, we create a performance-space plot for contacting layers on a given absorber material. For a simulated high-lifetime SnS absorber, this 2D performance-space illustrates two maxima, one local and one global. The local maximum occurs over a wide range of contact-layer doping concentrations (below 1016 cm−3), but only a narrow range of conduction band offsets (0 to −0.1 eV), and is highly sensitive t...

Non-monotonic effect of growth temperature on carrier collection in SnS solar cells

We quantify the effects of growth temperature on material and device properties of thermally evaporated SnS thin-films and test structures. Grain size, Hall mobility, and majority-carrier concentration monotonically increase with growth temperature. However, the charge collection as measured by the long-wavelength contribution to short-circuit current exhibits a non-monotonic behavior: the collection decreases with increased growth temperature from 150 °C to 240 °C and then recovers at 285 °C. Fits to the experimental internal quantum efficiency using an opto-electronic model indicate that the non-monotonic behavior of charge-carrier collection can be explained by a transition from drift- to diffusion-assisted components of carrier collection. The results show a promising increase in the extracted minority-carrier diffusion length at the highest growth temperature of 285 °C. These findings illustrate how coupled mechanisms can affect early stage device development, highlighting the critical role of direct...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm(2). Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Improving the Carrier Lifetime of Tin Sulfide via Prediction and Mitigation of Harmful Point Defects

Tin monosulfide (SnS) is an emerging thin-film absorber material for photovoltaics. An outstanding challenge is to improve carrier lifetimes to >1 ns, which should enable >10% device efficiencies. However, reported results to date have only demonstrated lifetimes at or below 100 ps. In this study, we employ defect modeling to identify the sulfur vacancy and defects from Fe, Co, and Mo as most recombination-active. We attempt to minimize these defects in crystalline samples through high-purity, sulfur-rich growth and experimentally improve lifetimes to >3 ns, thus achieving our 1 ns goal. This framework may prove effective for unlocking the lifetime potential in other emerging thin-film materials by rapidly identifying and mitigating lifetime-limiting point defects.

A path to 10% efficiency for tin sulfide devices

We preform device simulations of a tin sulfide (SnS) device stack using SCAPS to define a path to 10% efficient devices. We determine and constrain a baseline device model using recent experimental results on one of our 3.9% efficient cells. Through a multistep fitting process, we find a conduction band cliff of -0.2 eV between SnS and Zn(O,S) to be limiting the open circuit voltage (VOC). To move towards a higher efficiency, we can optimize the buffer layer band alignment. Improvement of the SnS lifetime to >1 ns is necessary to reach 10% efficiency. Additionally, absorber-buffer interface recombination must be suppressed, either by reducing recombination activity of defects or creating a strong inversion layer at the interface.

Phase-pure evaporation of tin (II) sulfide for solar cell applications

Tin (II) sulfide is a promising earth-abundant thin-film solar absorber material due to its strong absorption and near-optimal bandgap. We demonstrate phase-pure evaporation of SnS in a CdTe-like manufacturing process, achieving phase-pure SnS thin-films through thermal evaporation of SnS powder. We investigate the effects of SnS film thickness and growth rate on film morphology and correlate results with device performance. Working devices are achieved with SnS film thicknesses as low as 370 nm and growth rates of up to 50 Å/s, with efficiencies ranging from 1.1% to 2.6% in as-grown films.

Analysis of loss mechanisms in Ag2ZnSnSe4 Schottky barrier photovoltaics

Recently, a new type of photovoltaic device based on an n-type Ag2ZnSnSe4 (AZTSe) absorber was demonstrated with an efficiency of over 5%. This work examines in detail several critical loss mechanisms in FTO/AZTSe/MoO3/ITO Schottky barrier devices. It is shown that the Schottky barrier height in the present devices under illumination is only ∼0.82 eV. With a barrier height of this magnitude, the device has an upper-bound of only 6.4% to the efficiency. Second, the AZTSe thin films used in the present devices exhibit exceptionally short minority carrier lifetimes (≤225 ps), as probed using two-photon spectroscopy. Third, hysteresis is observed in the J–V response of these devices. Pulsed electrical measurements reveal that reverse-bias voltage pulses create residual shunts in the device proportional to the pulse amplitude and duration. Reversible Ag-doping of the MoO3 buffer is proposed as an explanation of the observed shunting upon reverse bias. Therefore, improving the contact materials and the minority...

Economically sustainable scaling of photovoltaics to meet climate targets

To meet climate goals, photovoltaics (PV) deployment will have to grow rapidly over the next fifteen years. We identify two barriers to this growth: scale-up of manufacturing capacity and the cost of PV module production. We explore several technoeconomic approaches to overcoming these barriers and identify deep reductions in the capital intensity (capex) of PV module manufacturing and large increases in module efficiency as the most promising routes to rapid deployment. Given the lag inherent in rolling out new technology, we explore an approach where growth is fueled by debt or subsidies in the short-term and technological advances in the medium term. Finally, we analyze the current capex structure of crystalline silicon PV module manufacturing to identify potential savings.