UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 21)*

Lighting Science Group Corporation

(Name of Issuer)

Common Stock, par value $.001 per share

(Title of Class of Securities)

53224G103

(CUSIP Number)

Richard Weinberg

LED Holdings, LLC

c\o Pegasus Capital Advisors, L.P.

99 River Road

Cos Cob, CT 06807

(203) 869-4400

(Name, Address and Telephone Number of Person Authorized to

Receive Notices and Communications)

December 16, 2011

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.    ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Amendment No. 21 to Schedule 13D

This Amendment No. 21 amends and supplements the Schedule 13D (the “Schedule 13D”) filed on behalf of LED Holdings, LLC (“LED Holdings”), PP IV (AIV) LED, LLC (“PPAIV”), PP IV LED, LLC (“PPLED”), Pegasus Partners IV, L.P. (“PPIV”), LSGC Holdings LLC (“LSGC Holdings”), LSGC Holdings II LLC (“LSGC Holdings II”), PCA LSG Holdings, LLC (“PCA Holdings”) Pegasus Investors IV, L.P. (“PIIV”), Pegasus Investors IV GP, L.L.C. (“PIGP”), Pegasus Capital, LLC (“PCLLC”), and Craig Cogut (“Mr. Cogut”) with the Securities and Exchange Commission (the “SEC”), as the case may be, on October 15, 2007, as amended by Amendment No. 1 filed on April 11, 2008, Amendment No. 2 filed on May 1, 2008, Amendment No. 3 filed on July 30, 2008, Amendment No. 4 filed on January 12, 2009, Amendment No. 5 filed on February 20, 2009, Amendment No. 6 filed on May 22, 2009, Amendment No. 7 filed on August 17, 2009, Amendment No. 8 filed on September 1, 2009, Amendment No. 9 filed on March 8, 2010, Amendment No. 10 filed on March 24, 2010, Amendment No. 11 filed on April 28, 2010, Amendment No. 12 filed on May 14, 2010, Amendment No. 13 filed on July 2, 2010, Amendment No. 14 filed on July 16, 2010, Amendment No. 15 filed on November 5, 2010, Amendment No. 16 filed on December 28, 2010, Amendment No. 17 filed on February 2, 2011, Amendment No. 18 filed on February 18, 2011, Amendment No. 19 filed on May 26, 2011 and Amendment No. 20 filed on December 13, 2011. Except as specifically provided herein, this Amendment No. 21 supplements, but does not modify any of the disclosure previously reported in the Schedule 13D and the amendments referenced above. Each capitalized term used but not defined herein has the meaning ascribed to such term in the Schedule 13D, as amended.

Item 3. Source and Amount of Funds or Other Consideration

The disclosure in Item 4 below is incorporated herein by reference.

Item 4. Purpose of Transaction

On December 16, 2011, the Issuer issued 6,500 units (the “Series G Units”) of the Issuer’s securities to LSGC Holdings II at a price per Series G Unit of $1,000.00 and total consideration of $6,500,000. Each Series G Unit consists of: (i) one share of the Issuer’s Series G Preferred Stock (the “Series G Preferred Stock”) and (ii) 83 shares of the Issuer’s common stock, par value $0.001 per share (the “Common Stock”). The Issuer issued the Series G Units pursuant to an option (the “Option”) to purchase Series G Units granted to LSGC Holdings II in connection with that certain Series G Unit Subscription Agreement (the “Series G Subscription Agreement”), dated as of December 1, 2011, by and among the Issuer, LSGC Holdings II, PCA Holdings, PPIV, Ensemble Lights, LLC, Belfer Investment Partners L.P., Lime Partners, LLC, Mr. Mark Kristoff and Mr. Alan Docter.

Pursuant to the terms of the Series G Subscription Agreement, on December 23, 2011, PCA Holdings assigned a portion of its Option to purchase the Series G Units under the Series G Subscription Agreement to an additional investor (the “Additional Investor”) for no consideration and that Additional Investor executed a joinder to the Series G Subscription Agreement and partially exercised the Option by electing to purchase 2,000 Series G Units. As a result of the partial exercise of the Option and purchase of Series G Units by LSGC Holdings II described above and the assignment of the Option by PCA Holdings and the purchase of Series G Units by the Additional Investor, the Option held by PCA Holdings, PPIV and LSGC Holdings II was reduced to an option to purchase 5,892 Series G Units.

PCA Holdings currently anticipates assigning an additional portion of the Option in accordance with its terms.

Participation Agreement

In connection with the Subscription Agreement (the “Series F Subscription Agreement”) dated as of November 17, 2011, by and among the Issuer, PCA Holdings and PPIV, and the Series G Subscription Agreement, PCA Holdings, PPIV, PIIV, Pegasus Capital Partners IV, L.P. and Pegasus Partners IV (OA), LP on December 19, 2011, entered into a Participation Agreement (the “Participation Agreement”) that for no consideration granted to certain limited partners of PPIV options to purchase any Series G Units subscribed for by PCA Holdings under the Series G Subscription Agreement, including the 10,000 Series G Units that were issued to PCA Holdings on December 1, 2011, upon the conversion of 10,000 Series F Units that were purchased previously under the Series F Subscription Agreement, and the 1,650 Series G Units that were issued to PCA Holdings on December 9, 2011, upon its partial exercise of the Option. The options may be exercised at any time on or prior to May 17, 2012, at a price per Series G Unit equal to the Stated Value (as defined in the Series G Certificate of Designation dated December 1, 2011) of the Series G Preferred Stock underlying such Series G Unit plus any accrued but unpaid dividends thereon. Upon a conversion of Series G Units into other securities pursuant to the terms of the Series G Subscription Agreement, the options set forth in the Participation Agreement will apply to such new securities. The Participation Agreement will terminate if the Series G Preferred Stock held by PCA Holdings is redeemed by the Issuer.

The foregoing description of the Participation Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Participation Agreement, which is included as Exhibit 10.1 to this Amendment No. 21 and is incorporated by reference herein.

The Reporting Persons continuously assess the Issuer’s business, financial condition, results of operations and prospects, general economic conditions, other developments and additional investment opportunities. Depending on such assessments, the Reporting Persons may acquire additional shares of Common Stock or may determine to purchase, sell or otherwise dispose of all or some of the Issuer’s securities in the open market, as applicable, in privately negotiated transactions or otherwise. Such actions will depend upon a variety of factors, including, without limitation, current and anticipated future trading prices, the financial condition, results of operations and prospects of the Issuer, alternative investment opportunities, general economic, financial market and industry conditions and other factors that the Reporting Persons may deem material to its investment decision.

Except as set forth herein or in previous amendments to this Schedule 13D, as the case may be, the Reporting Persons do not have present plans or proposals at this time that relate to or would result in any of the transactions described in subparagraphs (a) through (j) of Item 4 of Schedule 13D.

Item 5. Interest in Securities of the Issuer

Items 7 through 11 and 13 of each of the cover pages of this Amendment No. 21 are incorporated herein by reference. Such information is based upon 204,769,385 shares of Common Stock outstanding as of December 16, 2011, which includes all shares outstanding as of December 16, 2011, and all shares issued pursuant to the Series G Subscription Agreement as discussed herein.

The disclosure regarding the Participation Agreement and the related disclosure in Item 4 is incorporated herein by reference.

Except as set forth herein and in our previously filed amendments to Schedule 13D, as the case may be, there have been no other transactions in the class of securities reported on that were effected within the past sixty days.

Item 6. Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer

The disclosure regarding the Participation Agreement in Item 4 is incorporated herein by reference.

The Participation Agreement is included as Exhibit 10.1 to this Amendment No. 21 and is incorporated herein by reference.

Item 7. Material to be Filed as Exhibits

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: December 23, 2011

Exhibit 10.1

PARTICIPATION AGREEMENT

THIS PARTICIPATION AGREEMENT (this “Agreement”) is entered into as of December 19, 2011, among PCA LSG Holdings, LLC, a Delaware limited liability company (“PCA Holdings”), Pegasus Partners IV, L.P., a Delaware limited partnership (“Fund IV”), Pegasus Investors IV, L.P., a Delaware limited partnership (the “Fund IV GP”), Pegasus Capital Partners IV, L.P., a Delaware limited partnership (“PCP IV”), and Pegasus Partners IV (OA), LP, a Delaware limited partnership (the “OA Fund”). Capitalized terms used but not defined herein shall have the meanings ascribed thereto in the Second Amended and Restated Agreement of Limited Partnership of Fund IV dated as of September 28, 2007, as amended.

WHEREAS, Lighting Science Group Corporation, a Delaware corporation (the “Company”), a Portfolio Company of Fund IV, is currently marketing a private placement investment opportunity, but has received interim funding from, among others, PCA Holdings, an Affiliate of Fund IV GP;

WHEREAS, pursuant to a Subscription Agreement among the Company, PCA Holdings and Fund IV, dated as of November 17, 2011 and attached as Exhibit A (the “Series F Subscription Agreement”), the Company granted PCA Holdings the option to purchase, in the aggregate, up to 40,000 units (the “Series F Units”) of the Company’s securities at a purchase price of $1,000.00 per Series F Unit, each Series F Unit consisting of: (a) one share of the Company’s Series F Preferred Stock, par value $0.001 per share; and (b) 83 shares of the Company’s common stock, par value $0.001 per share, and PCA Holdings purchased 10,000 of the Series F Units;

WHEREAS, pursuant to a Subscription Agreement among the Company, PCA Holdings, Fund IV and certain other Persons, dated as of December 1, 2011 and attached as Exhibit B (the “Series G Subscription Agreement”), the Company has issued to certain other Persons units (“Series G Units”) of the Company’s securities at a purchase price of $1,000 per Series G Unit, with each Series G Unit consisting of: (a) one share of the Company’s Preferred Stock, par value $0.001 per share (the “Preferred Stock”), issued pursuant to the Certificate of Designation attached as Exhibit C; and (b) 83 shares of the Company’s common stock, par value $0.001 per share;

WHEREAS, in connection with the issuance of Series G Units by the Company, pursuant to Section 7 of the Series F Subscription Agreement, PCA Holdings exchanged its outstanding Series F Units for Series G Units; provided that the Preferred Stock associated therewith shall begin earning and accruing dividends as of November 17, 2011;

WHEREAS, effective as of December 9, 2011, PCA Holdings exercised its right under the Series G Subscription Agreement to purchase an additional 1,650 Series G Units from the Company at a purchase price of $1,000 per Series G Unit;

WHEREAS, the Advisory Board of Fund IV has given its consent to PCA Holdings providing interim funding to the Company; and

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WHEREAS; PCA Holdings now wishes to enter into certain arrangements regarding the Series G Units with the other parties hereto;

NOW, THEREFORE, in consideration of the mutual agreements herein contained, the parties hereto agree as follows:

(a) PCA Holdings hereby agrees that at any time on or prior to May 17, 2012, each Limited Partner of Fund IV may elect to purchase some or all of its pro rata share of all Series G Units purchased by PCA Holdings based on such Limited Partner’s Capital Commitment relative to the total Capital Commitments of all Limited Partners. Upon an exchange of securities pursuant to Section 8 of the Series G Subscription Agreement or a Subsequent Transaction (as defined in the Series G Subscription Agreement), the offer set out herein shall apply to such new securities (the Series G Units or such other securities, the “Offered Units”). The offer shall terminate if the Preferred Stock is redeemed by the Company, notwithstanding notification of the Limited Partners.

(b) Fund IV shall notify each Limited Partner of its right to elect to purchase the Offered Units: (i) promptly following the date of this Agreement; and (ii) following any additional subscription for Series G Units by PCA Holdings. Fund IV shall also notify each Limited Partner in the event that the Preferred Stock is redeemed by the Company and the offer is therefore terminated.

(c) Each notice sent pursuant to Section 1(b) above shall (i) specify the number of Offered Units held by PCA Holdings as of such date and each Limited Partner’s pro rata share of such Offered Units; (ii) state that a Limited Partner electing to purchase Offered Units (an “Electing LP”) must give written notice to PCA Holdings of its intention to purchase Offered Units, including the number of Offered Units it wishes to purchase; (iii) state the Purchase Price (as defined below); and (iv) state that an election to purchase may be made at any time on or prior to May 17, 2012, subject to an earlier redemption of the Preferred Stock.

(d) PCA Holdings shall transfer the relevant Offered Units to an Electing LP within 10 Business Days of receipt of such Electing LP’s confirmation of its election, subject to the Electing LP entering into a purchase agreement with PCA Holdings and PCA Holdings’ receipt of the Purchase Price. The Purchase Price shall be paid by the Electing LP by wire transfer of immediately available funds to PCA Holdings on the date of transfer.

(e) The “Purchase Price” for the Offered Units shall be $1,000 per Offered Unit plus any dividends accrued but unpaid on the Preferred Stock (or any other preferred stock or debt security issued pursuant to a Subsequent Transaction or any other exchange of securities pursuant to the Series G Subscription Agreement) through the date that such Offered Units are transferred.

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(a) PCA Holdings shall contribute to Fund IV promptly upon receipt of any Participation Amount the product of (i) the Investor Percentage and (ii) such Participation Amount.

(b) As used in this Section 2, the following terms shall have the following meanings:

“Assumed Tax Rate” shall mean the applicable highest marginal tax rates for an individual resident in New York City (including the Medicare tax on investment income of individuals) applicable to ordinary income, qualified dividend income or capital gains, as appropriate, taking into account year in which the taxable net income is received, and taking into account the deductibility of state and local income taxes as applicable at the time for United States federal income tax purposes.

“Investor Percentage” shall mean 100% less the percentage of Fund IV’s total Capital Commitments represented by the Fund IV GP, PCP IV and the OA Fund.

“Participation Amount” shall mean 50% of the excess of (i) all amounts paid by the Company in respect of the Offered Units over (ii) $1,000 per Offered Unit plus a dividend of 10% per annum, on a cumulative and compounded basis, but net of taxes payable on any such amounts calculated at the Assumed Tax Rate.

(c) Each of the Fund IV GP, PCP IV and the OA Fund hereby waives its right to receive any portion of the Participation Amount. Accordingly, the Participation Amount shall be distributed as an Other Distribution among the other Limited Partners.

(d) For the avoidance of doubt, the calculation of the Participation Amount shall remain unchanged notwithstanding that: (i) a higher dividend becomes payable on the Preferred Stock in accordance with the terms thereof, or (ii) the Series G Units are exchanged for any other securities pursuant to Section 8 of the Series G Subscription Agreement or a Subsequent Transaction, and such new securities may receive a higher dividend than 10% per annum.

(a) Third Party Rights. The Limited Partners of Fund IV shall be third party beneficiaries of this Agreement, and this Agreement shall not be amended in a manner adverse to the Limited Partners without the consent of the Advisory Board.

(b) Governing Law. The laws of the State of New York without reference to the conflict of laws provisions thereof shall govern all questions concerning the construction, validity and interpretation of this Agreement.

(c) Amendment and Waiver. The provisions of this Agreement may be amended and waived only with the prior written consent of each of the parties hereto.

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(d) Counterparts. This Agreement may be executed in counterparts, each of which shall be an original and all of which shall constitute a single agreement.

[Signature page follows]

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IN WITNESS WHEREOF, the parties hereto have executed this Agreement to be effective as of the date first written above.

Exhibit 99.1

AGREEMENT REGARDING THE JOINT FILING OF SCHEDULE 13D

The undersigned hereby agree as follows:

(i) Each of them is individually eligible to use the Schedule 13D to which this Exhibit is attached, and such Schedule 13D is filed on behalf of each of them; and

(ii) Each of them is responsible for the timely filing of such Schedule 13D and any amendments thereto, and for the completeness and accuracy of the information concerning such person contained therein; but none of them is responsible for the completeness or accuracy of the information concerning the other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate.

Date: December 23, 2011