Solution-processed solar cells are particularly suited to benefit from absorption enhancement by plasmonic nanoparticles, due to their transport-limited film thicknesses and the ease with which metal nanoparticles can be integrated into the materials. Despite practical demonstrations of performance enhancements, the overall benefits have so far been limited in scope to photocurrents well below the theoretical limits. In this Perspective, we critically evaluate the prospects for plasmonic enhancements in solution-processed thin-film solar cells. We give an overview of recent work, focusing on embedded plasmonic nanoparticles in organic, perovskite, and colloidal quantum dot solar cells. We then develop an intuitive effective medium model for embedded plasmonic nanostructures in photovoltaic thin films, evaluate the model in the context of previous results in the field, and use the model to provide a framework for identifying the most promising avenues for realizing plasmonic performance enhancements in solution-processed solar cells. Our results indicate that further plasmonic enhancement gains may be possible in organic photovoltaic cells, whereas concentrating on improving transport in perovskite and colloidal quantum dot architectures is a more promising route to performance advances. Additionally, fine-tuning the concentration of plasmonic enhancers within the absorbing medium is critical for achieving maximum photocurrent potential.

The strong and counterproductive interrelationship of thermoelectric parameters remains a bottleneck to improving thermoelectric performance, especially in polymer-based materials. In this paper, we investigate a compositional range over which there is decoupling of the electrical conductivity and Seebeck coefficient, achieving increases in at least one of these two parameters while the other is maintained or slightly increased as well. This is done using an alkylthio-substituted polythiophene (PQTS12) as additive in poly(bisdodecylquaterthiophene) (PQT12) with tetrafluorotetracyanoquinodimethane (F4TCNQ) and nitrosyl tetrafluoroborate (NOBF4) as dopants. The power factor increased two orders of magnitude with the PQTS12 additive at constant doping level. Using a second pair of polymers, poly(2,5bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTTC12) and poly(2,5-bis(3dodecylthiothiophen-2-yl)thieno[3,2-b]thiophene, (PBTTTSC12), with higher mobilities, we also observe decoupling of the Seebeck coefficient and electrical conductivity, and achieve higher power factor. Distinguished from recently reported works, these two sets of polymers possess very closely offset carrier energy levels (0.05 ~ 0.07 eV), and the microstructure, assessed using grazing incidence X-ray scattering, and mobility evaluated in field-effect transistors, was not adversely affected by the blending. Experiments, calculations and simulations are consistent with the idea that blending and doping polymers with closely-spaced energy levels and compatible morphologies to promote carrier mobility favors increased power factors.

We report on the significant performance enhancement of SnO2 thin film ultraviolet (UV) photodetectors (PDs) through incorporation of CuO/SnO2 p-n nanoscale heterojunctions. The nanoheterojunctions are self-assembled by sputtering Cu clusters that oxidize in ambient to form CuO. We attribute the performance improvements to enhanced UV absorption, demonstrated both experimentally and using optical simulations, and electron transfer facilitated by the nanoheterojunctions. The peak responsivity of the PDs at a bias of 0.2 V improved from 1.9 A/W in a SnO2-only device to 10.3 A/W after CuO deposition. The wavelength-dependent photocurrent-to-dark current ratio was estimated to be ~ 592 for the CuO/SnO2 PD at 290 nm. The morphology, distribution of nanoparticles, and optical properties of the CuO/SnO2 heterostructured thin films are also investigated.

Colloidal quantum dots (CQDs), are a promising candidate material for realizing colored and semitransparent solar cells, due to their band gap tunability, near infrared responsivity and solution-based processing flexibility. CQD solar cells are typically comprised of several optically thin active and electrode layers that are optimized for their electrical properties; however, their spectral tunability beyond the absorption onset of the CQD layer itself has been relatively unexplored. In this study, we design, optimize and fabricate multicolored and transparent CQD devices by means of thin film interference engineering. We develop an optimization algorithm to produce devices with controlled color characteristics. We quantify the tradeoffs between attainable color or transparency and available photocurrent, calculate the effects of non-ideal interference patterns on apparent device color, and apply our optimization method to tandem solar cell design. Experimentally, we fabricate blue, green, yellow, red and semitransparent devices and achieve photocurrents ranging from 10 to 15.2 mA/cm² for the colored devices. We demonstrate semitransparent devices with average visible transparencies ranging from 27% to 32%, which match our design simulation results. We discuss how our optimization method provides a general platform for custom-design of optoelectronic devices with arbitrary spectral profiles.

Although record efficiencies in colloidal quantum dot (CQD) solar cells continue to increase, they are still demonstrated on impractically small-area devices. Concentrators can effectively enlarge the active area, allowing scaled-up energy harvesting. Here, we present an economical and scalable method to fabricate compact concentrators made from polydimethylsiloxane using 3D-printed molds, which are directly bonded to CQD solar cells. The resulting integrated systems deliver more than a 20-fold increase in photocurrent and power, as well as significant open circuit voltage enhancements, over the original cells. We use the integrated systems to identify limiting factors in CQD solar cell operation under high irradiance. Our method could pave the way to making practical high-power solution-processed solar cells.