Objective

Design and demonstrate a combined materials-, device-, and module-based engineering approach to creating environmentally stable perovskite solar cell modules.

Description

Perovskite solar cells (PSCs) are an increasingly promising photovoltaic (PV) technology, as their power conversion efficiency has increased from less than 4% at the outset of research in 2009 to over 25% today [1 – 4]. Metal halide and hybrid perovskites adopt the general ABX3 chemical formula and crystallize in the perovskite structure, where the A-site is typically occupied by an organic cation like methylammonium or an alkali ion like Cs, the B-site is occupied by a metal cation like Pb, and the X-site is occupied by a halide ion like Cl.

This class of perovskites exhibits strong light absorption and emission, has excellent electronic transport characteristics, and is amenable to solution-processing methods. These advantages may translate to significant improvements in PV size, weight, power, and cost (SWaP-C), which could enable the US Army to efficiently generate electrical power from the sun in a variety of environments ranging from large permanent installations to Soldier-level power-on-the-move.

Despite these advantages, poor thermodynamic stability, hygroscopic behavior, and poor environmental stability continually plagues PSCs and is limiting their development and ultimate technological impact.

This challenge is manifold: lead halide perovskites themselves are thermodynamically unstable with respect to decomposition (i.e., they have a positive enthalpy of formation) [5]; high mobility of X-ions causes significant ion migration during PSC operation and degrades material quality and PV performance; thermal stresses and thermal cycling during operation further degrade performance; and the presence of humidity during PV operation ultimately destroys crystal quality and PV module performance over long periods.

These problems are compounded by a lack of mechanistic understanding of degradation modes. Thus, a holistic research effort is needed to improve stability across the PSC hierarchy, ranging from fundamental science and engineering at the materials level, to device engineering, to module design and integration. This scope-encompassing effort would provide (a) better insight into the physics and chemistry of perovskite degradation; (b) new materials design rules that imbue perovskites with resistance to thermodynamic instability and ion migration; (c) device engineering approaches spanning contacts/electron transport layer/hole transport layer/substrate that address interfacial, thermal, and moisture instability; and (d) module engineering approaches that mitigate or eliminate sources of instability (e.g., moisture, thermal regulation) that cannot otherwise be addressed with materials design or device engineering approaches.

Recent isolated, limited-scope research advances suggest this approach is feasible—for example, perovskite A- and B-site ion composition can be tuned to improve stability at the materials and device level [6]. Likewise, composition and tolerance factor engineering in oxide [7] and hybrid perovskites [8] suggests that entropy may be an underutilized tool for thermodynamic stability, i.e., an “entropy-stabilized” hybrid perovskite [9,10]. Interfacial ion-blocking barriers in devices may be useful to modulate chemical potential to suppress ion migration [11]. Ionic passivation of grain boundaries may also suppress ion migration [12]. Encapsulation strategies at the device and module level can provide added protection against humidity and thermal cycling, though more work is needed [13].

Phase I

Design a concept for an environmentally stable perovskite solar cell module that incorporates stability science and engineering at the materials and thermodynamic stability level, device level, and the module/packaging level. Describe the proposed thermodynamics and materials design science, device engineering, and module packaging schemes that will be employed.

Perform ab initio atomistic modeling, molecular dynamics simulations, thermodynamic calculations, electromagnetic simulations, finite element analysis, and/or technology computer-aided design (TCAD) as needed to demonstrate the feasibility of the proposed approach. The module design must have a minimum of 400-square-cm PV-active area and consist of four (4) individual 100-square-cm perovskite solar cells wired in series, parallel, or combination thereof.

The module must be designed to have an absolute power conversion efficiency of 15% or greater. The module must be designed to retain 90% or more of its initial power conversion efficiency over an 8000-hour period while being subjected to 1 Sun, 40˚C, and 85% relative humidity (RH) for at least 4000 hours. Outline the techniques and procedures that will be used to fabricate the proposed design and characterize its PV power conversion performance.

Outline the necessary techniques and procedures specifically needed to evaluate PSC environmental stability based on, or appropriately adapted from, the International Summit on Organic PV Stability (ISOS) [14]. Proposed stability tests must include, but are not limited to, shelf-life and dark-storage testing, outdoor testing, light-soaking testing, thermal cycling testing, and combined light-humidity-thermal cycling testing. The proposed model solution must elucidate the stability parameters requirements, stability constraints, and demonstrably meet the elements critical to success of the proposed design.

A critical Phase I deliverable is to create at least one physical module prototype that successfully demonstrates one or more of the stabilized solutions that are critical to success of the proposed model design. This prototype must demonstrate one or more of the proposed stabilization approaches: improved perovskite materials thermodynamic stability, device engineering, and/or the module integration scheme.

This physical module prototype must have at least 100-square-cm PV-active area and a power conversion efficiency of 7.5% or greater. The prototype must retain 75% or more of its initial power conversion efficiency over a 720-hour period while being subjected to 1 Sun, 40˚C, and 85% relative humidity (RH) for at least 360 hours.

Phase II

Based on the designs, modeling, and prototypes from Phase I, fabricate, test, and demonstrate at least one operational PSC-based solar cell module. The module must have a minimum of 400-square-cm PV-active area and consist of four (4) individual 100-square-cm perovskite solar cells wired in series, parallel, or combination thereof. The module must have a power conversion efficiency of 15% or greater.

Perform the proposed ISOS testing protocols and any additional tests, as appropriate, to characterize the solar module stability. Using accelerated and/or surrogate testing methods, environmental chambers, and/or field testing, demonstrate that the prototype module will retain 90% or greater of its initial power conversion efficiency over 8000 hours when subjected to 1 Sun illumination and the entire range of climactic operating conditions (i.e., 11 different daily cycles in air temperature and relative humidity) defined in Table 3-1 of AR 70-38 [15].

Data and metrics to report must include initial solar cell characterization (current-voltage curve, maximum power point, internal and external quantum efficiency), encapsulation strategy and performance (wiring, layering, edge sealing, geometry, evolution of stresses/strains within these components), aging conditions (electrical bias, cycling, light, temperature, atmosphere), number of samples, outdoor stability, and, importantly, the evolution of power conversion efficiency over time (i.e., how long until the module efficiency degrades to 90% of its maximum power output or peak efficiency?).

Phase III

Phase III will transition the newly developed stabilized PSC module technology to commercial availability through prime contractors that build integrated solar power systems, the original equipment manufacturers that manufacture PV modules, other relevant suppliers, and/or other partnering agreement(s), as appropriate.

Commercialization of this technology may occur via the incorporation of one or more stabilization approaches anywhere in the PV module (e.g., materials design, device engineering, module integration, etc.).

Ideally, a successful effort will deliver a capability upgrade for a relevant Army Program of Record at the end of Phase III, in the form of a solar power generating system capable of providing power against SWaP-C metrics of $3/W or less, 150 W/kg or more, and a functional lifetime of 5 years or greater. Expected dual-use applications include commercial PV power plants, self-charging electric vehicles, microgrids for self-powering infrastructure components, residential solar power, and portable solar power generators and battery chargers.

Submission Information

Please refer to the 23.B BAA for more information. Proposals must be submitted via the DoD Submission site at https://www.dodsbirsttr.mil/submissions/login