W.L. Rance

Bulk heterojunction organic photovoltaic devices based on phenyl-cored thiophene dendrimers

Bulk heterojunction organic photovoltaic devices have been fabricated by blending phenyl-cored thiophene dendrimers with a fullerene derivative. A power conversion efficiency of 1.3% under simulated AM1.5 illumination is obtained for a four-arm dendrimer, despite its large optical band gap of 2.1eV. The devices exhibit an increase in short-circuit current and power conversion efficiency as the length of the arm is increased. The fill factors of the devices studied are characteristically low, which is attributed to overly uniform mixing of the blend.

Photoinduced Carrier Generation and Decay Dynamics in Intercalated and Non-intercalated Polymer:Fullerene Bulk Heterojunctions

The dependence of photoinduced carrier generation and decay on donor-acceptor nanomorphology is reported as a function of composition for blends of the polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C(14)) with two electron-accepting fullerenes: phenyl-C(71)-butyric acid methyl ester (PC(71)BM) or the bisadduct of phenyl-C(61)-butyric acid methyl ester (bis-PC(61)BM). The formation of partially or fully intercalated bimolecular crystals at weight ratios up to 1:1 for pBTTT-C(14):PC(71)BM blends leads to efficient exciton quenching due to a combination of static and dynamic mechanisms. At higher fullerene loadings, pure PC(71)BM domains are formed that result in an enhanced free carrier lifetime, as a consequence of spatial separation of the electron and hole into different phases, and the dominant contribution to the photoconductance comes from the high-frequency electron mobility in the fullerene clusters. In the pBTTT-C(14):bis-PC(61)BM system, phase separation results in a non-intercalated structure, independent of composition, which is characterized by exciton quenching that is dominated by a dynamic process, an enhanced carrier lifetime and a hole-dominated photoconductance signal. The results indicate that intercalation of fullerene into crystalline polymer domains is not detrimental to the density of long-lived carriers, suggesting that efficient organic photovoltaic devices could be fabricated that incorporate intercalated structures, provided that an additional pure fullerene phase is present for charge extraction.

14%-efficient flexible CdTe solar cells on ultra-thin glass substrates

Flexible glass enables high-temperature, roll-to-roll processing of superstrate devices with higher photocurrents than flexible polymer foils because of its higher optical transmission. Using flexible glass in our high-temperature CdTe process, we achieved a certified record conversion efficiency of 14.05% for a flexible CdTe solar cell. Little has been reported on the flexibility of CdTe devices, so we investigated the effects of three different static bending conditions on device performance. We observed a consistent trend of increased short-circuit current and fill factor, whereas the open-circuit voltage consistently dropped. The quantum efficiency under the same static bend condition showed no change in the response. After storage in a flexed state for 24 h, there was very little change in device efficiency relative to its unflexed state. This indicates that flexible glass is a suitable replacement for rigid glass substrates, and that CdTe solar cells can tolerate bending without a decrease in device performance.

Low-bandgap thiophene dendrimers for improved light harvesting

This article follows our previous work on the synthesis and characterization of pi-conjugated dendrimers for use in organic solar cells. Here we discuss five new thiophene-based dendrimers that were synthesized in order to study the relationship between their chemical structures and electronic properties. Three of these dendrimers incorporate acetylene spacers, included to relieve steric strain, between the thiophene arms and phenyl cores used in previous studies. Only a small effect on the electronic properties is observed upon inclusion of the acetylene spacer in the three-arm dendrimer, 3G1-2S-Ac. In contrast, a decrease in the bandgap is observed for the four-arm dendrimer, 4G1-2S-Ac, due to a reduction of interactions between the arms in the more sterically congested 1,2,4,5-arrangement around the phenyl core, resulting in delocalization of the exciton through the phenyl core. Incorporation of electron-withdrawing cyano groups on the phenyl core of the three-arm dendrimer, 3G1-2S-CN, resulted in a very large (∼0.5 eV) decrease in the bandgap, due to stabilization of the lowest unoccupied molecular orbital, and the low energy absorption band in this material is attributed to a transition with significant intramolecular charge-transfer character. The electronic properties of three- and four-arm dendrimers with electron-donating dibutylaniline moieties attached to the end of the thiophene dendron, 3G1-2S-N and 4G1-2S-N respectively, are almost identical, indicating that they are dominated by the arms, with no through-core communication allowed, even for the para-linked arms of 4G1-2S-N. However, there is a significant increase in the molar absorptivity of these materials, concomitant with significant broadening of the absorption spectrum, which is an important attribute in light-harvesting applications.

The effect of CdTe growth temperature and ZnTe:Cu contacting conditions on CdTe device performance

CdTe device performance is strongly dependent on the quality of the back contact and the ability of the back contact to introduce a copper doping profile in the CdTe layer itself. Copper-doped ZnTe (ZnTe:Cu) is a nearly ideal contact material for CdTe solar cells due to its work function and ability to source copper to CdTe. Most of the ZnTe:Cu studies in the past used CdTe grown at relatively low deposition temperatures (550°C and below). Here we investigate the use of ZnTe:Cu as a back contact for CdTe grown at temperatures up to 620°C. We observe a strong interplay between the CdTe absorber deposition conditions and optimized ZnTe:Cu contacting conditions. Device JV characteristics suggest that CdTe solar cells with absorber layers deposited by close-space sublimation (CSS) at high temperature, 600-620°C, are more robust to the back contact Cu doping level and contacting temperature than CdTe grown at lower temperatures. The implication for industrial processes is a ~1% absolute increase in device efficiency for devices in which the CdTe is deposited on PV glass at high temperature. Perhaps more importantly, this increased performance is maintained for a larger window of temperature and doping level of the ZnTe:Cu back contact. For devices with CdTe absorbers deposited at 600°C, device efficiency in excess of 13.5% is maintained for back contacts containing 2-5 wt.% Cu, and for contacting temperatures ranging from 300-360°C. Red-light bias quantum efficiency (QE) and capacitance-voltage (CV) measurements are used to probe the effect of the introduced copper doping profiles and net acceptor density to better understand how ZnTe:Cu sources influences the resulting CdTe device.

Properties of oxygenated cadmium sulfide (CdS:O) and their impact on CdTe device performance

In this work, we report on the development of a reactive sputtering process for CdS:O for high efficiency CdTe solar cells. X-ray diffraction, UV-Vis-NIR spectrophotometry, and Rutherford backscattering spectrometry were used to characterize the crystal structure, composition, and optical properties, respectively. All films were slightly Cd-rich, while the bulk oxygen content increased up to 45 at. % in direct proportion to the O2 partial pressure. Optical absorption in cells was reduced by increasing the oxygen fraction in the sputtering ambient. Optimal performance was obtained from cells with CdS sputtered in a 6% O2/Ar ambient, yielding efficiency >14% and VOC >840 mV.

The use of Corning® Willow™ glass for flexible CdTe solar cells

New flexible glass products should enable the fabrication of high efficiency, flexible CdTe devices because of their high optical transmission and compatibility with the high temperature processing conditions often used for making high performance CdTe solar cells. Here, we will report on our preliminary results using Corning® Willow™ Glass in a high temperature CdTe device fabrication process. For all device studies, we used MOCVD deposited SnO2:F/SnO2 bilayers as the transparent conducting oxide/buffer. We investigated CdS window layers deposited by both room temperature sputtering and chemical bath deposition (CBD). Using 550°C CdTe layers deposited by close-spaced sublimation (CSS) on both types of CdS layers, we made CdTe devices with efficiencies above 12% with Willow Glass. These efficiencies are comparable to identically processed devices on rigid glass, confirming that Willow Glass is compatible with all high temperature CdTe processing steps.

High-efficiency flexible CdTe superstrate devices

Flexible, superstrate CdTe devices combine the advantages of a commercially demonstrated, low-cost manufacturing process with a lightweight, flexible form factor. Here, we present data on cell efficiencies greater than 16%, and the critical processing changes that have enabled recent efficiency increases. The devices in this study were made on Corning® Willow® Glass, which is a highly transparent, flexible, hermetic, and dimensionally stable substrate that can withstand high processing temperatures. To date, we have produced devices with several different combinations of front and back contacts on this glass and have found that it is compatible with most of our standard processing steps. One of our best devices to date has a certified efficiency of 16.2%, with a short-circuit current density (Jsc) of 25.6 mA/cm2, an open-circuit voltage of 820 mV, and a fill factor (FF) of 77.3%. The increased Jsc in this cell is due to an improved sputtered CdS:O deposition process, and the high FF is due to a co-evaporated ZnTe:Cu back contact.

Performance of transparent conductors on flexible glass and plastic substrates for thin film photovoltaics

High-performance transparent conductive indium-tin-oxide (ITO) films on flexible glass have been flextested to 25-50k bend cycles without breakage, and with ~0.1% change in sheet resistance. In contrast, commercial ITO/PET samples undergo ~50-100% increase in sheet resistance in the same test, indicating that such coatings/substrates may not be acceptable for use in some products or fabrication procedures. The flexible glass substrate enables high-temperature processing, which facilitates the high performance of the coatings. Measurements of the volume resistivity and water vapor transmission rate (WVTR) indicate that Corning® Willow® Glass is suitable as a PV substrate material without need for barrier coatings or glass lamination.

Response of CdS/CdTe devices to Te exposure of back contact

Theoretical predictions of thin-film CdS/CdTe photovoltaic (PV) devices have suggested performance may be improved by reducing recombination due to Te-vacancy (VTe) or Te-interstitial (Tei) defects. Although formation of these intrinsic defects is likely influenced by CdTe deposition parameters, it also may be coupled to formation of beneficial cadmium vacancy (VCd) defects. If this is true, reducing potential effects of VTe or Tei may be difficult without also reducing the density of VCd. In contrast, post-deposition processes can sometimes afford a greater degree of defect control. Here we explore a post-deposition process that appears to influence the Te-related defects in polycrystalline CdTe. Specifically, we have exposed the CdTe surface to Te prior to ZnTe:Cu/Ti contact-interface formation with the goal of reducing VTe but without significantly reducing VCd. Initial results show that when this modified contact is used on a CdCl2-treated CdS/CdTe device, significantly poorer device performance results. This suggests two things: First, the amount of free-Te available during contact formation (either from chemical etching or CuTe or ZnTe deposition) may be a more important parameter to device performance than previously appreciated. Second, if processes have been used to reduce the effect of VTe (e.g., oxygen and chlorine additions to the CdTe), adding even a small amount of Te may produce detrimental defects.