DOI: 1.138/NMAT3984 Supplementry Informtion for Improved performnce nd stility in quntum dot solr cells through nd lignment engineering Chi- Ho M. Chung 1, Ptrick R. Brown 2, Vldimir Bulović 3 & Moungi G. Bwendi 4* 1 Deprtment of Mterils Science nd Engineering, 2 Deprtment of Physics, 3 Deprtment of Electricl Engineering nd Computer Science, 4 Deprtment of Chemistry, Msschusetts Institute of Technology, Cmridge, Msschusetts, 2139, United Sttes. *emil: mg@mit.edu NATURE MATERIALS www.nture.com/nturemterils 1
DOI: 1.138/NMAT3984 wvelength (nm) 6 8 1 12 14 Asornce or PL (.u) 1.5 1..5. Asornce PL 2. 1.8 1.6 1.4 1.2 1..8 photon energy (ev) 1-Trnsmittnce (%) c 1 9 8 7 ZnO thin-film on glss 6 5 4 3 2.8.6.4.2 1 ZnO solution. 3 4 5 6 7 Wvelength (nm) Asornce (.u.) Figure S1 Mterils chrcteriztion., Asorption nd photoluminescence spectr of PS QDs in octne solution., Asorption spectr of ZnO nnocrystl solution nd ZnO thin film on glss sustrte. The sorption onset t wvelength λ~37nm corresponds to ndgp of ~3.35eV. The ckground t longer wvelengths in the thin film sorption spectrum is due to light scttering nd reflection. c, TEM imge of ZnO nnocrystls (scle r: 2nm). 2 NATURE MATERIALS www.nture.com/nturemterils
DOI: 1.138/NMAT3984 SUPPLEMENTARY INFORMATION 1-Trnsmittnce (%) 5 45 4 35 3 25 PS-TBAI As-mde Air 1dy Air 3dys Air 5dys Air 7dys Air 2dys 2 1-Trnsmittnce (%) 4 6 8 1 12 14 Wvelength (nm) 5 45 4 35 3 25 2 PS-EDT As-mde Air 1dy Air 3dys Air 5dys Air 7dys Air 2dys 4 6 8 1 12 14 Wvelength (nm) Figure S2 Evolution of sorption spectr of PS thin films (2 lyers) with ir storge time., The sorption spectr of the PS- TBAI film shows no pprent chnge, indicting good stility., The sorption spectr of the PS- EDT film shows monotonic decrese in intensity nd lue shift with ir storge time s result of oxidtion. The discontinuity t wvelength λ=8nm is n rtifct resulting from detector chngeover. NATURE MATERIALS www.nture.com/nturemterils 3
DOI: 1.138/NMAT3984 Current Density (ma/cm 2 ) -5-1 -15-2 -25 A: TBAI12 B: TBAI1/EDT 2 C: TBAI 6/EDT 6 D: TBAI 2/EDT1 E: EDT12..1.2.3.4.5.6.7 Voltge (V) c.65 25 V OC (V).6.55.5.45.4 A B C D E 1 2 3 4 5 6 7 Air storge dys J SC (ma/cm 2 ) 2 15 1 5 1 2 3 4 5 6 7 Air storge dys Figure S3 Comprison of devices with different numers of PS- TBAI nd PS- EDT lyers., Representtive J- V curves., nd c, The chnge of V OC nd J SC with ir storge time, respectively. The device structure is ITO/ZnO/PS- TBAI(x)/PS- EDT(y)/MoO 3/Al, where x nd y denote the numers of PS- TBAI nd PS- EDT lyers. The totl numer of PS lyers is 12 lyers in ll devices. The PS- EDT- only device (Device E) exhiits the lowest J SC ut highest V OC mong ll devices. It lso shows decresing J SC, nd n incresing V OC with ir- exposure time. By replcing the top 2 PS- TBAI lyers with PS- EDT lyers (Device B), the J SC significntly incresed compred to the PS- TBAI- only device (Device A). We conclude tht the PS- TBAI lyer is the min sorer lyer nd the improvement in J SC in PS- TBAI/PS- EDT devices shown in Fig 1 is not from dditionl light sorption from the PS- EDT lyer. This cn e further supported y the progressively decresing J SC with decresing numers of TBAI lyers in devices B to D. 4 NATURE MATERIALS www.nture.com/nturemterils
DOI: 1.138/NMAT3984 SUPPLEMENTARY INFORMATION Ultrviolet Photoelectron Spectroscopy (UPS) UPS is used to determine the Fermi level (EF) nd the vlence nd mximum (EV) with respect to vcuum level (EVAC) of the PS thin films. The sic principles nd n exmple spectrum re shown in Fig. S4. For photoelectron to escpe the smple surfce nd to e collected, it hs to hve sufficient energy to overcome the sum of the inding energy (with respect to EF) of its initil level nd the work function (Φ), where Φ = EE!"# EE!. Therefore, for fixed incident photon energy of 21.2 ev, the secondry electron cut- off (high inding energy edge) represents photoelectrons with zero kinetic energy (Ek) when they escpe the smple surfce nd their initil level is shown s the grey dotted line inside the DOS in Fig. S4. The work function Φ is determined y the difference etween the incident photon energy (21.2 ev) nd the inding energy of the secondry electron cut- off. In the exmple spectrum (Fig. S4), the cut- off inding energy is 16.43 ev s determined y the intersection of the liner portion of the spectrum nd the seline. The work function of this smple is thus Φ=21.2-16.43=4.77 ev; tht is, EF is - 4.77 ev with respect to EVAC. The difference etween EF nd EV is determined y the intersection of the liner portion of the spectr ner the Fermi edge (low inding energy region) with the seline. The exmple spectrum hs EF - EV=.82 ev. Therefore, its vlence nd mximum EV is - 5.59 ev with respect to EVAC. The conduction nd minimum (EC) is further clculted y dding the opticl ndgp, s determined y the position of the lowest exciton sorption pek, to EV. Figure S4 Bsic principles of UPS mesurements., Schemtic illustrtion of the energy digrm. DOS: density of sttes in the vlence nd., An exmple UPS spectrum. The left pnel shows the mgnified view of the high inding energy region nd the right pnel shows the mgnified view of the low inding energy region. Grey lines represent the liner fits. NATURE MATERIALS www.nture.com/nturemterils 5
DOI: 1.138/NMAT3984 Bnd lignment etween PS- TBAI nd PS- EDT Thickness- dependent UPS is used to determine the nd lignment etween PS- TBAI nd PS- EDT. The thickness of the thickest PS- EDT overlyer corresponds to the thickness generlly used in devices nd is determined to e ~45nm y profilometer. This thickest PS- EDT lyer ws otined in the sme mnner s for device friction (2 lyer- y- lyer spin- coting steps from 5 mg/ml PS solution followed y lignd exchnge with EDT solution). The thinner PS- EDT lyers (4.5 nm, 9 nm, 13.5 nm) were otined y spin- coting dilute PS solution (1 mg/ml) on PS- TBAI. The thicknesses of these thinner PS- EDT lyers were estimted y the solution concentrtion (1 mg/ml, 5 times less thn for the thickest lyer) nd the numers of lyer- y- lyer spin- coting steps (1, 2, nd 3 steps, respectively). As shown in Fig. S5, two distinct peks from PS- EDT (~3.8eV nd ~5.8eV) pper in the UPS spectr fter we dd PS- EDT on PS- TBAI. The secondry electron cut- off shifts to higher inding energy while the EF - EV decreses with incresing thickness of PS- EDT. These fetures indicte interfcil nd ending t the PS- TBAI/PS- EDT interfce. The fitted nd positions re plotted in Figure 2 nd summrized in Tle S1. We note tht the sturtion of the spectrl shpe nd the nd positions t ~13.5nm lso confirms tht the ~45nm of PS- EDT lyer used in our photovoltic devices is thick enough to result in continuous overlyer nd its thickness my e eyond the width of the interfcil nd ending region. Intensity (.u) 45 nm 13.5 nm 9 nm E F thickness of PS-EDT on PS-TBAI 4.5 nm nm 17 16 2 15 1 5 Binding energy (ev) 2 1 Figure S5 UPS spectr of PS- TBAI films covered with different thicknesses of PS- EDT. The left pnel shows the secondry electron cut- off region nd the right pnel shows the mgnified spectr ner Fermi edge. Spectr were shifted for clrity. 6 NATURE MATERIALS www.nture.com/nturemterils
DOI: 1.138/NMAT3984 SUPPLEMENTARY INFORMATION Tle S1. Bnd positions with respect to vcuum s determined from the UPS spectr in Fig S5. The error rs in E F- E V represent the error from fitting. Smple EC EF EV EF- EV TBAI (11nm) - 4.26-4.77-5.59.82±.2 TBAI (11nm)/ EDT (4.5nm) - 4.1-4.73-5.46.7±.2 TBAI (11nm)/ EDT (9.nm) - 3.78-4.41-5.11.7±.2 TBAI (11nm)/ EDT (13.5nm) - 3.68-4.38-5.1.63±.2 TBAI (11nm)/ EDT (45.nm) - 3.68-4.38-5.1.63±.2 The thickness- dependent UPS dt llow us to determine interfcil nd ending t the PS- TBAI/PS- EDT interfce. At equilirium, the Fermi level in the whole smple must lign. Therefore, the reltive nd lignment cn e determined y mtching the Fermi level nd then plcing the nd edge energies with respect to the Fermi level. It cn e etter understood with the help of the schemtic illustrtions shown in Figure S6. The left figure shows the experimentlly determined nd positions with respect to vcuum level s function of the thickness (dn) of PS- EDT on PS- TBAI. The reduction of work function fter dding the PS- EDT lyer implies downwrd vcuum level shift, wheres the reduction of EF - EV implies n upwrd nd ending from the interfce to PS- EDT. Once the thickness of the PS- EDT is greter thn the width of the interfcil nd ending region, the nd positions nd the UPS spectr rech sturtion (d2 nd d3). The right figure shows the nd lignment deduced from the left figure. Figure S6 Schemtic illustrtions explining how to determine the nd lignment t the interfce from the UPS spectr., The nd positions s function of the overlyer thickness (dn). Here, the conduction nd edge (E C), vlence nd edge (E V) nd vcuum levels (E VAC) re referenced to the Fermi level (E F). IE: ioniztion energy., The corresponding nd lignment. For simplicity, the conduction nd nd vlence nd re connected y stright lines. NATURE MATERIALS www.nture.com/nturemterils 7
DOI: 1.138/NMAT3984 As control experiment, we compre the UPS dt of PS- TBAI film nd PS- TBAI film soked in EDT solution (Fig. S7). Soking the PS- TBAI film in EDT solution without depositing new PS- EDT lyer does not chnge the work function significntly ut slightly decreses the vlence nd offset (EF- EV) from.82 ev to.75 ev. Therefore, simply soking the PS- TBAI film in EDT solution is not enough to give rise to significnt conduction nd offset (vlence nd offset) tht cn lock electron flow (fcilitte hole extrction). Furthermore, the fetures from PS- EDT (~3.8 ev nd 5.8 ev) do not pper in the EDT- soked PS- TBAI film, suggesting tht soking PS- TBAI films in EDT solution cnnot chnge it into PS- EDT. PS-TBAI PS-TBAI, EDT-soked -3.5-4. PS-TBAI PS-TBAI EDT-soked Intensity (.u) Energy (ev) -4.5-5. -5.5 17 16 2 15 1 5 Binding energy (ev) 2 1-1 -6. Figure S7 UPS spectr of PS- TBAI film nd PS- TBAI film soked in EDT solution without depositing new PS- EDT lyer. The right figure shows the corresponding nd positions with respect to vcuum. 8 NATURE MATERIALS www.nture.com/nturemterils
DOI: 1.138/NMAT3984 SUPPLEMENTARY INFORMATION Current Density (ma/cm 2 ) 14 12 1 8 6 4 2-2 As-mde Air 1dy Air 2dys Air 4dys Air 6dys -4-1. -.8 -.6 -.4 -.2..2.4.6.8 1. Voltge (V) Current Density (ma/cm 2 ) 1 5-5 -1-15 -2-25 As mde 1 dy 2 dys 4 dys 6 dys -3 -.1..1.2.3.4.5.6 Voltge (V) Figure S8 Development of S- shpe J- V chrcteristics in PS- TBAI/PS- EDT device with MoO 3/Au node fter ir- exposure. shows the mgnified view of. 5 5 Current Density (ma/cm 2 ) -5-1 -15-2 -25 As mde Air 5 mins Air 1 dy Current Density (ma/cm 2 ) -5-1 -15-2 -25 As mde Air 1 hr Air 1 dy -3 -.1..1.2.3.4.5.6 Voltge (V) -3 -.1..1.2.3.4.5.6 Voltge (V) Figure S9 Exmples of initil increse of device performnce fter short ir- exposure time fter evportion of the metl electrodes. The device structure is ITO/ZnO/PS- TBAI/PS- EDT/Au. nd were fricted in the sme conditions ut on different sustrtes. The devices were fricted nd stored in ir overnight efore node deposition under vcuum. The ir- exposure time shown here represents further ir- exposure time fter node evportion under vcuum. NATURE MATERIALS www.nture.com/nturemterils 9
DOI: 1.138/NMAT3984 Figure S1 Certified record efficiency to dte for colloidl quntum dot solr cells. This unencpsulted device hd een stored in ir for 37 dys efore eing tested in ir y n ccredited lortory. 1 NATURE MATERIALS www.nture.com/nturemterils
DOI: 1.138/NMAT3984 SUPPLEMENTARY INFORMATION Current Density (ma/cm 2 ) 3 2 1-1 -2 Mesured in ir Mesured in N 2-3 -.6 -.4 -.2..2.4.6 Voltge (V) Figure S11 J- V chrcteristics of n unencpsulted PS- TBAI/PS- EDT device with Au node mesured in ir nd under inert N 2- tmosphere. No significnt difference in performnce ws found when the device ws mesured in ir. The slight difference is ttriuted to the different solr simultors used for ech mesurement nd other experimentl uncertinties. 1 Current Density (ma/cm 2 ) 5-5 -1-15 -2-25 TBAI/EDT/Au Device re: 1.24 mm 2 5.44 mm 2-3 -.1..1.2.3.4.5.6 Voltge (V) Figure S12 J- V chrcteristics of devices with different device res. To test the sclility nd reduce the experimentl error in determining device re, we fricted devices with device re of 5.44 mm 2, ~4 times lrger thn our typicl devices (1.24 mm 2 ). The figure plots the J- V curves of nine devices with 1.24 mm 2 nd six devices with 5.44 mm 2 re. Devices with lrger device re show similr performnce. NATURE MATERIALS www.nture.com/nturemterils 11
DOI: 1.138/NMAT3984 Figure S13 Certified performnce for high fill fctor device with lrger re. This device ws certified fter 131 dys of ir storge nd showed the highest fill fctor of 66.7% in QD solr cells to dte. The nominl totl device re for this device is 5.44 mm 2 s defined y the overlp of the node nd cthode. For the certifiction, 3 mm 2 msk ws ttched to the device to define the device re. 12 NATURE MATERIALS www.nture.com/nturemterils
DOI: 1.138/NMAT3984 SUPPLEMENTARY INFORMATION Figure S14 Histogrms of J SC nd power conversion efficiency of devices with MoO 3 nodes. The histogrms show the performnce of devices on different sustrtes from different tches using the sme size of PS QDs. The PS- TBAI/PS- EDT devices consistently outperform the PS- TBAI devices in every tch of devices we fricted. Figure S15 Histogrms of power conversion efficiency of PS- TBAI/PS- EDT devices with Au nodes. The histogrm shows the efficiency of more thn 3 devices with Au nodes on different sustrtes. Unlike the devices with MoO 3 nodes whose ir- stility vry due to the uncontrolled mient humidity, ll of the devices with Au node exhiit excellent ir stility. NATURE MATERIALS www.nture.com/nturemterils 13
DOI: 1.138/NMAT3984 3. 2.5 n, PS-TBAI 1..8 n 2. 1.5 1. n, PS-EDT.6.4 k Normlized Electric field intensity E 2 1.8 1.6 1.4 1.2 1.8.6.4.5. E field instensity in device ITO ZnO PS TBAI k, PS-TBAI.2 k, PS-EDT. 4 6 8 1 12 14 16 18 Au Wvelength (nm) 45 nm 6 nm 75 nm 9 nm c Normlized Electric field intensity E 2 1.8 1.6 1.4 1.2 1.8.6.4 E field instensity in device ITO ZnO PS TBAI PS EDT 45 nm 6 nm 75 nm 9 nm Au.2.2 d 1 2 3 4 5 6 Position in Device (nm) e 1 2 3 4 5 6 Position in Device (nm) 12 4.E+19 12 4.E+19 11 3.2E+19 11 3.2E+19 1 2.4E+19 1 2.4E+19 Wvelength (nm) 9 8 7 6 5 1.6E+19 8.E+18 8.E+15 Wvelength (nm) 9 8 7 6 5 1.6E+19 8.E+18 8.E+15 4 4 3 2 4 6 8 1 12 14 16 18 2 22 3 2 4 6 8 1 12 14 16 18 2 22 Position (nm) Position (nm) Figure S16 Opticl modeling results., the complex refrctive indices of PS- TBAI nd PS- EDT films s determined y ellipsometry. nd c, modeled electric filed intensity in the devices for four selected wvelengths. d nd e, modeled photon sorption rte (1/sec- cm 3 ) in the PS- TBAI min soring lyer under 1- sun AM1.5G illumintion. nd d, PS- TBAI- only device; c nd e, PS- TBAI/PS- EDT device. The position t nm represents the ZnO/PS- TBAI junction. The horizontl stripes re due to the dips in the AM1.5G solr spectrum (O 2 nd H 2O sorption). The results show tht the opticl field is similr in PS- TBAI- only nd PS- TBAI/PS- EDT devices. No new opticl modes developed in the PS- TBAI/PS- EDT devices. Moreover, significnt portion of the longer wvelength photons (>5nm) is sored deeper in the film, i.e. close to the PS- TBAI/PS- EDT interfce. Opticl modeling ws performed y using model nd codes from literture S1. 14 NATURE MATERIALS www.nture.com/nturemterils
DOI: 1.138/NMAT3984 SUPPLEMENTARY INFORMATION Reference S1. Burkhrd, G. F., Hoke, E. T. & McGehee, M.D. Accounting for interference, scttering, nd electrode sorption to mke ccurte internl quntum efficiency mesurements in orgnic nd other thin solr cells. Adv. Mter. 22, 3293 (21) ; code: http://www.stnford.edu/group/mcgehee/trnsfermtrix/index.html. NATURE MATERIALS www.nture.com/nturemterils 15